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Published in the Rivista internazionale del diritto comune 15 (2004) 57-76.  A revised version was published in Law as Profession and Practice in Medieval Europe: Essays in Honor  of James A. Brundage, edited by Kenneth Pennington and Melodie Harris Eichbauer (Ashgate 2011) 93-115.


The Formation of the Jurisprudence of the Feudal Oath of Fealty


Kenneth Pennington




Good Samaritan Luke 10.25-37


Fulbert of Chartres (1006-1028)


Huguccio's Duty to Assist


Moderatio inculpatae tutelae

Hugo Grotius, De iure belli et pacis Deutsches Strafgesetzbuch § 323c

Code pénal  § 63

James A. Brundage

In homagio et fidelitate

When I studied with James Brundage forty years ago he was exploring the importance of the crusader vow in medieval canon law. He thought that historians had neglected law, especially canon law, as a tool for understanding the role of the crusade in medieval society. His first foray into legal history produced a splendid study of the crusading vow and of its jurisprudence that has convinced many crusade historians that they should pay attention to the legal ramifications of the institutions of the crusade. Footnote Although earlier crusade historians had not considered canon law to be an important element in the history of the crusades before Brundage’s work, in retrospect it is not surprising that canon law regulated crusader vows and other institutions of the crusade. Vows were important in the life of the Church and in secular society.

            In almost all human societies oaths embedded in rituals create social bonds. Footnote Medieval society was no exception. The feudal oath of homage and fealty that a vassal took to his lord is almost emblematic of the popular and scholarly image of medieval social and political relationships. In the Liber consuetudinum Mediolani, a compilation of the customs of Milan that was promulgated in 1216, there is an oath that the vassal should take to his lord: Footnote

I, <James>, swear that henceforward I will a faithful man or vassal to my lord. I will not lay open to another what he has entrusted to me in the name of fealty to <my lord’s> injury.

The text of the custom enigmatically concludes: “Many things are contained in these words, which are difficult to insert here.” Footnote The sentence would have been puzzling, however, only to those who did not know feudal law. A thirteenth-century jurist reading this text would have recognized immediately that the compilers of the customs were referring to a letter of Bishop Fulbert of Chartres (1006-1028) that Gratian had included in his Decretum. He would have understood that the reference was not just to Fulbert’s letter but to the canonistic glosses and commentaries that circled the letter in the margins of manuscripts. He would have known that Fulbert’s letter was incorporated into the Libri feudorum, the standard text of feudal law for the next four centuries. Footnote If he had been well-trained in canon law he might even have thought about the twelfth-and early thirteenth-century canonists, especially Huguccio and Tancred, who had shaped the jurisprudential doctrines of this central institution of feudal law: the feudal oath. That fact might seem remarkable to us but not to him. In his world legal systems had very permeable borders.

            By 1216 Fulbert’s letter had become the most important legal text for defining the oath of homage and fealty. The letter’s origins lie in a request that William V, count of Poitou and duke of Aquitaine made to Fulbert asking for a clarification of the obligations and duties that a vassal owed to a lord. William had troubled relationships with his vassals. In his reply (ca. 1020) Fulbert wrote a short treatise on the ethics of feudal relationships that circulated fairly widely. Footnote Some later canonists wondered why Gratian included the letter in his Decretum. Footnote It seemed out of place. But a number of canonical collections before Gratian had thought the text appropriate for their collections, and its future as a fundamental legal text was assured when Bishop Ivo of Chartres (1091-1116) included it in his Panormia. Footnote Ivo treated the crime of perjury in the section of his collection in which he incorporated the letter. Gratian treated clerical oaths in Causa 22. He placed Fulbert’s letter near the end of the Causa when he placed it in the first recension of his Decretum (C.22 q.5 c.18) ca. 1125. It became a locus classicus for canonistic discussions of the feudal contract and the norms that regulated the relationship of a lord and vassal. Footnote

            Fulbert told William that when a vassal took an oath to his lord six things were understood to be contained in it whether explicitly expressed or not: to keep his lord safe, to protect him from harm, to preserve the lord’s justice, to prevent damage to his possessions, and to assist and not hinder his lord in the carrying out of his duties. Fulbert alleged that he got this list from written authorities, but his exact source, if there were one, has never been discovered. For the next four centuries jurists cited Fulbert’s list of obligations and duties as being central to the feudal oath of fealty. The text in the first recension of Gratian’s Decretum reads: Footnote

The form of fidelity that anyone may owe to a lord and vice versa, may be found in a letter of Bishop Fulbert.

Since I was asked to write something about the oath of fidelity, I have noted for you these things which follow from the authority of books. Whoever swears fidelity to his lord should always have six things in mind: safe, secure, honest, useful, easy, possible. Safe, namely, lest he injure his lord with his own body. Secure lest he not injure his secret interests or his defenses through which his lord can be secure. Honest lest he not injure his lord’s justice or in other matters which seem to pertain to his honesty. Useful lest he injure his lord’s possessions. Easy or possible, lest that the good, which his lord could easily do, he would make difficult, and that what would be possible, he would make impossible for his lord. A faithful man should pay heed to these examples. Footnote

It is not sufficient to abstain from evil, unless he may do what is good. It remains that he faithfully give his lord counsel and help in the aforementioned matters, if he wishes to be worthy of his benefice (fief) and safe in the fidelity that he has sworn. The lord also ought to render his duty to his faithful man in all things. If he does not, he may be thought of as faithless, just as he, who in consenting or telling lies will be perfidious and perjurious. Huguccio

The wording of the text is interesting. The count had asked Fulbert to discuss the responsibilities of anyone who swore an oath of faithfulness to a lord. Fulbert does not use the word “vassal” to describe this person, and we should not assume that the count was asking about what would be called a feudal oath. At the end of his letter Fulbert calls this person a “fidelis.” Since oaths were part of the social fabric of society, Fulbert’s letter could be used to define the legal status of anyone who swore an oath of faithfulness or fealty to another.

            The canonists did not pay much attention to this text at first. Perhaps to them it seemed to be an appendix to Causa 22 in which Gratian discussed clerical oaths. In their eyes it might have served as an introduction to Causa 23 treating the rules of war. They did not seem to think that Fulbert’s admonitions, which pertained primarily to laymen, were grist for their mill. Johannes Faventinus (ca. 1170) was one of the first canonists to comment on Fulbert’s letter. He noted that vassals did not have to obey lords who had been excommunicated. Footnote He also did a clever bit of textual exegesis. He explained that Fulbert did not use the word “periurus” to describe a lord who did not honor his commitment to his vassal but “malefidus.” The reason was, Johannes explained, that a lord did not normally swear an oath to a vassal. Footnote Consequently he could be accused only of bad faith, not of perjury. Only the oathtaker could commit perjury. By the end of the feudal age lords still did not swear oaths of fealty to their vassals. Footnote

            Huguccio (ca. 1190) was the first canonist to give Fulbert’s letter a close and extensive reading. At the beginning of his commentary he noted that many things are tacitly understood when someone took an oath, vow, and promise. Footnote He then discussed each of the six tacit obligations listed by Fulbert. Huguccio interpreted the oath of fidelity by placing its obligations against the norms of the jurisprudence in the Ius commune. If there were cause or reason (causa et ratio) a vassal could injure his lord. Cause and reason were, perhaps, the most powerful norms in medieval jurisprudence and generally trumped any rule, law, custom, or statute. Footnote For example, if a vassal were a judge or a magistrate — a social situation into which only urban vassals would probably fall — he could punish his lord if he merited it. Footnote According to Huguccio, Fulbert’s principle of honesty encompassed two points. A vassal could not injury a lord’s justice or his women. First he observed that according to customary law, even though it was unwritten, a vassal could not testify against his lord in court. Again he looked to other norms of the Ius commune to qualify the prohibition. If justice and cause demanded it, the vassal could testify against him if his lord promoted an unjust court case. Then the lord “had no justice.” Footnote Huguccio also included sexual morality under the rubric of honesty. Perhaps because he had read too many French lais about the sexual misconduct of the nobility, he defined vassal’s honesty as not violating the women who surrounded his lord. The lord’s wife and daughter were, understandably, not to be touched. Huguccio, however, wanted to protect all women from the depredations of the nobility. He included any other woman who lived in the lord’s home. In sum, the vassal should not do any dishonest thing in his lord’s house. Footnote This may be another example of Huguccio’s propensity to embrace moral absolutes, what later canonists called the “rigor of Huguccio.” Footnote In any case later canonists like Johannes Teutonicus included only his lord’s wife and daughter. Footnote

            Huguccio discussed a vassal’s obligation to give his lord counsel and help extensively. His first point was the vassal was only obligated to give aid when the lord needed help in licit and honest affairs. If his lord was injured a vassal should respond immediately, but within reasonable limits (moderatio inculpatae tutelae) and with attention to the admonition of Saint Paul in Romans 12:19: an enemy should be treated with respect; disarm malice with kindness. Footnote The concept of justifiable defense that Huguccio cited (moderatio inculpatae tutelae) was taken from Roman law and slowly penetrated the Ius commune during the twelfth century. Footnote Huguccio’s combining of Roman and Biblical precepts to establish a legal norm was typical of twelfth-century jurists. Footnote See Pennington, "Moderamen inculpatae tutelae: The Jurisprudence of a Justifiable Defense," Rivista Internazionale di Diritto Comune 24 (2013) 27-55

            Huguccio then turned to the question of the moral and legal responsibility of a vassal to defend others. Nobody should sin himself or for another, he reflected, but at the same time everyone has an obligation to defend anyone from injury. Footnote Huguccio’s presumption was a commonplace for him and other continental jurists but was contrary to the historical norms of British and American common law where the doctrine of nonfeasance has held sway to the present day that required no duty-to-assist. Footnote Under the influence of the Ius commune and especially under the influence of the doctrine in feudal law governing the oath of fealty, most civil law legal systems have a duty-to-assist other persons in their jurisprudence. Footnote Grotius

            Huguccio had no doubt that every man had a duty to assist another person. This duty was not limited to those who had sworn the oath of fealty in Christian society. If this were true, he wondered, what is the legal force behind the vassal’s duty to help his lord? How would a vassal’s duty to his lord differ from his duty to aid others in distress? Footnote He found the answer to that question in a conciliar canon: “I say that the vassal is bound to his lord <by the oath of fealty> more willingly and more specially — just as in the conciliar canon from the Council of Toledo in Gratian’s Decretum. That canon stated that the breaking of promises is to be feared.” Footnote Huguccio quoted a phrase from the canon and expected that his readers would supply the complete quotation: “<the breaking of> specific promises is more to be feared than <the breaking of> of general vows.” Footnote Later canonists followed Huguccio’s lead and insisted that a vassal must do more than defend his lord just when he is in danger. Alanus Anglicus (ca. 1200) formulated a lapidarian expression of the precept: “Although the oath of fealty does not expressly state it, a vassal should give heed that his lord may not be injured.” Footnote Tancred (ca. 1215) and following him, Bernardus Parmensis in the Ordinary Gloss (ca. 1245), insisted that persons who swore oaths of fealty and obedience must protect their lords from attack and harm. They were also bound to protect them from plots and dangerous plans. Footnote This principle remained an important part of the jurisprudence that informed the oath of fidelity.

            Huguccio then turned to a vassal’s military obligation to aid his lord. He formulated several hypotheticals. What if the lord wishes to seize his vassal’s fief or property? The vassal must not obey his lord unless his lord’s war were just. A vassal was not bound to obey if his lord moved against him personally. Footnote What, however, if the lord moved against his son or his father? Huguccio’s answer relied on juridical distinctions drawn for the family, kin, and vassals of excommunicates. Footnote The vassal did not have to obey his lord when his son and father lived under the same roof. Otherwise, if his lord were waging a just war against his family, the vassal was held to obey his lord. Footnote

            Fulbert’s letter had laid down the norms that a vassal must adhere to if he were worthy of his fief. If a vassal violated these principles and showed himself unworthy, his lord could take his fief (beneficium) away from him. Footnote Significantly Huguccio applied the rules governing a vassal’s loss of his fief to the ecclesiastical sphere. What if, he asked, a cleric offered legal protection and assistance (patrocinium) to someone who had brought a court case against his own church or against his bishop to whom he has sworn fidelity? Huguccio thought that the cleric should lose his benefice unless he was pursuing his own legal case or that of his own people. He concluded by noting that while their lords are excommunicated, those who have sworn oaths of loyalty are not compelled to obey them. Footnote

            As I have already noted earlier, Gratian took his text of Fulbert’s letter from a source that contained a puzzling sentence: A faithful man should pay heed to these examples ( Ut fidelis hec documenta [nocumenta Ivo’s Panormia and Decretum, Ed. Rom.] caveat, iustum est). If “documenta” were the correct word in the text, then “caveat” meant “pay heed to.” If “nocumenta (injuries)” were the original wording, then “caveat” meant “avoid” The sentence then meant: A faithful man should avoid these injuries. Huguccio was not aware of the textual variant. Nevertheless, he thought it necessary to define the somewhat unusual meaning of “documenta . . . caveat” in the letter for his readers and students. Footnote He glossed “caveat” twice. In the second gloss he made two points. First, if the vassal did not swear an oath that explicitly upheld the norms listed in Fulbert’s letter, he still was obligated by the duties and obligations of a person who had sworn an oath of fealty. He recognized that the feudal oath had many different forms in feudal law. Some were more detailed than others. His gloss was directed to the short and simple oaths like the one in the customs of Milan. A vassal might swear a simple oath, but Huguccio argued that his obligations were still complex. Second, and more importantly, Huguccio linked the oaths that vassals took to their lords to those that bishops, partriarchs, and primates took to the pope, that bishops took to their superiors, and that clerics took to their bishops. Footnote Linking clerical oaths of fealty to the feudal oath gave Fulbert’s letter a lasting and important place in canonical jurisprudence. Footnote

            Two key texts in the body of canon law established these oaths of fealty in the Church. The first was a papal decretal letter, Ego (Petrus) episcopus, attributed to Pope Gregory the Great but which seems to have been cobbled from eleventh-century canonical sources. It probably had its origins in the second half of the eleventh century and was most likely connected with Pope Gregory VII’s insistence that bishops swear obedience to him. Footnote Huguccio merged De forma fidelitatis and Ego (Petrus) episcopus into one. The rules that Fulbert formulated for the count of Aquitaine applied equally to the norms governing the oaths that bishops and clerics took to their superiors in the Church. The oath of obedience contained in the papal letter Ego (Petrus) episcopus provided a model that was used for clerical and lay oaths of fealty. When Pope Innocent III extracted this same oath from Count Ildebrando Aldobrandeschi, Basilius, archbishop of Trnovo, and King John of England, the core of all three oaths contained the same wording as that found in Ego (Petrus) episcopus. Footnote By the early thirteenth century, when a person took an oath of fidelity, he was bound by many of the same norms and fell under the same jurisprudential principles in both the secular and the ecclesiastical spheres. Huguccio’s connection of the feudal oath to the ecclesiastical oath of fealty should not surprise us. As we have seen from our discussion of his thought, he did not balkanize canon, Roman, and feudal laws as we balkanize our legal systems today. Huguccio believed that if principles were valid in one legal system they could be valid in another. That was an attitude that he shared with every jurist of the Ius commune. Footnote

            The canonists who wrote after Huguccio expanded upon the jurisprudence that he created for the oath of fealty. By importing another definition from Roman jurisprudence, Alanus commented that a vassal who betrayed his lord fell under the Roman law of treason. Footnote The jurists liked that connection. A number of them repeated it. Footnote Johannes Teutonicus copied this gloss into his Ordinary Gloss where it remained a principle of feudal law until the end of feudalism. The Roman law of treason specified the death penalty for the crime. The canonists turned a traitor from a perjurer into capital felon. It was no small step. They marked a stage in the development of law in which the rights and honor of the lord became identified with much more than just another person. He became the symbol of the territorial state. The Chansons de geste had long emphasized a warrior’s faithlessness as the ultimate betrayal (“trahison”) in a world of honor. Footnote At the beginning of the thirteenth century the jurists of the Ius commune followed the poets. Footnote

            Later canonists refined and developed Huguccio’s points. An anonymous canonist focused on the vassal’s duty to aid his lord in court. He knew that a vassal’s oath prevented him from testifying against his lord in court, especially when his lord’s lawsuit was unjust. A vassal, however, had a duty to everyone to testify in a just case. As Huguccio has already asked, how is the lord’s status different from anyone else? The canonist cited the opinions of other jurists who maintained that a vassal was obliged to help his lord in court for the slightest reason. He should help others only if there were great need. Footnote This canonist agreed with that argument. When he discussed a vassal’s duty to rend aid and counsel to his lord, he exclaimed: “Most of all he should aid his lord. He should help others but most of all his lord. Or say when in doubt a vassal must always help his lord; he can help others only when he knows that they fight a just war.” Footnote Finally, he concurred with Huguccio that just as a vassal would lose his fief for violating the terms of his oath, a cleric should lose his benefice. He extended the norm to a lawyer who had sworn an oath of loyalty to a city. If the lawyer broke faith with the city, he should lose his stipend. Footnote This is another example of how broadly the jurists interpreted the rules governing the taking of oaths and applied them to all institutions of society.

            Laurentius Hispanus added two final points. First he distinguished between the oath that a vassal took to his lord and that which a cleric took to his bishop. The cleric should not and cannot obligate himself to a feudal contract. The oath taken by the cleric is different, argued Laurentius, as is shown by the decretal Ego (Petrus) episcopus. Footnote In another gloss he noted that if a vassal violated his oath and his lord died, his lord’s heirs could still bring him to justice. Roman law proved that the right to punish faithless vassals was inheritable. Footnote

            Fulbert of Chartres’ letter in Gratian’s Decretum provided the canonists with an opportunity to step foot directly into the feudal world. The church had long used oaths of obedience, and, as we have seen, the canonists saw the ecclesiastical oath as an institution governed by the same rules as the secular oaths of fealty. They also knew that bishops were sometimes feudal lords. We have already seen that the decretal Ego (Petrus) episcopus played an important role in the feudal and ecclesiastical spheres. Footnote Three other decretals occupied a significant place in the jurisprudence surrounding the oath of fealty after they were included in canonical collections. In chronological order they were Pope Alexander III’s Ex dilgenti, Pope Lucius III’s Retulit, and Pope Clement III’s Veritatis. All three were either in Compilatio prima or secunda; Ex diligenti and Veritatis entered the official corpus of canon law compiled by Raymond of Peñafort for Pope Gregory IX in 1234. Raymond inserted a small but important part of Retulit, in the Decretals under the title De regulis iuris (Rules of law). Each decretal raised questions that had not be dealt with in the commentaries on the feudal oath in Gratian’s Decretum. Footnote

            The canonistic commentaries on Ex diligenti were the most important because they refined the crucial distinction between the ecclesiastical and secular oaths of fealty. Pope Alexander III had sent Ex diligenti to a cleric in Canterbury whose archdeacon had behaved badly. Footnote The cleric told the pope that the archdeacon had persuaded him with promises, pressure and the sollicitude of important persons to swear homage and fealty to him. In return the archdeacon gave him an ecclesiastical benefice each year. The cleric knew that his promise and obligation was illicit. He petitioned the pope to absolve him from this obligation. He promised that he would never accept another benefice from the archdeacon. Alexander looked favorably on the cleric’s request. He absolved him from his oath, enjoined him to receive an appropriate penance from a bishop or priest, and absolved him from any infamy that might still stigmatize his person.

            The issue that was raised in the letter, although not explicitly, was simony. If a cleric received an ecclesiastical office only because he swore homage and fealty to his prelate, he committed simony. To receive a fief for an oath of fealty in the secular realm was honorable, but it was not honorable or licit in the church. A cleric could and should swear an oath of obedience to his prelate. However, the canonists noted that he could not conclude a contract that bartered his obedience for a benefice. Footnote Pope Alexander had declared that the granting of a benefice under those terms violated divine and human laws. One canonist remarked that he never remembered reading that such a grant violated human laws. Footnote He was right. Canon law had not yet established rules for regulating the oath of obedience and for separating it from the bestowal of benefices. Alexander took the first step. Later Tancred answered the jurist’s question about human laws prohibiting clerics to render an oath quite unconvincingly with a very dubious citation to Roman law. Footnote He presented a precedent but not a good one. Alanus Anglicus made the most perceptive comment on the decretal. He noted that a feudal oath was dissolved when either the lord or the vassal broke the contract. By renouncing the oath the cleric had broken the contract. Consequently his renunciation dissolved the oath. Therefore, Alanus concluded, the pope should not have written in his decretal that “he absolved” the oath, but rather that he “declares it having been absolved.” Footnote The tacit point that Alanus makes is that, besides the problem of simony, a feudal oath of homage and fealty is quite different from the permanent oath of obedience to a prelate. Tancred underlined Alanus’ point when he glossed the word homage in a decretal of Pope Lucius III. Lucius repeated the conclusions of the canonists that clerics could not swear homage to their prelates for spiritual things. Tancred noted that homage included fealty. Clerics could not take an oath fealty to their prelate for a benefice. They could, however, take an oath of obedience. Footnote Bernardus Parmensis clarified the legal issue even further. In cases when a cleric ought to swear an oath of obedience, that is of fealty, to his prelate, he may do it after he had received his spiritual office. Footnote In Ex diligenti Alexander established that the feudal oath of homage and fealty was not suited for the church. A prelate could never have a feudal contract with his cleric even though the cleric’s oath of faithfulness was regulated by the same norms and rules as fealty and homage. Bernardus Parmensis incorporated Alanus’ comment into the Ordinary Gloss to the decretal in the Decretals of Gregory IX. The oath of homage to a cleric is foul and illcit. It does not bind the oathtaker in law or in fact because it is an oath that may be dissolved unilaterally or mutually. The bottom line for the canonists was that homage should not be demanded for spiritual things. Footnote

            The condemnation of ecclesiastical homage first appeared in canon law in the early 1180's. When Pope Lucius III (1181-1185) dealt with an unnamed English bishop who demanded strict obedience from his clerics, these legal issues were still in flux outside of the law schools. Footnote The bishop’s archdeacon appealed to the pope because the bishop attempted to usurp the customary and legal rights of his archdeaconal office. The bishop’s interference with the archdeacon’s rights continued even after he had appealed to Rome. Footnote The archdeacon reported, however, that the bishop seemed likely to cease his opposition if he would render homage to him and swear a new oath of fealty for his good will (pro sua voluntate). Footnote It is always dangerous to read between the lines of these decretals, but it is likely that a new bishop wanted to rein in an uppity archdeacon. Footnote

            In the decretal Retulit Lucius wrote to the bishops of York and the other judges delegate that if the archdeacon had sworn fealty to his previous bishop they were to order the new bishop to desist completely from demanding homage and fealty from him. Footnote The pope underlined his point by stating that it was completely contrary to the custom of the Roman church that anyone be compelled to swear homage for spiritual offices. Footnote Only this last section of the decretal became a permanent part of canon law. Lucius’ conclusion is completely consonant with contemporary legal thought. If the archdeacon had already sworn an oath of obedience to his bishop he would not be required to repeat his oath to a new bishop. Footnote The bishop could not demand homage because that institution of feudal law, with its implicit quid pro quo, a benefice for loyalty, was not appropriate in the ecclesiastical sphere. A few years later Pope Clement III (1188-1191) extended the canonical rule about repeating oaths to the vassals of prelates in the decretal Veritatis. Footnote King William II of Sicily had asked Clement whether his heirs must swear oaths of homage and fealty to all of Clement’s successors who succeeded to the papal throne. Clement’s answer was brief: No. Once the oaths were made, they could not be compelled to swear again. Tancred noted that freedman owing obedience to the church must repeat vows of loyalty within a year and a constitution of Frederick I dictated that a vassal must swear the oath of homage and fealty to his new lord within a year or he would lose his feudal rights. The decretal, Retulit, however, established that persons were not compelled to swear oaths again. Tancred resolved this conflict by leaving the question open. He conjectured that Clement might have given William a special indulgence or that the oaths of fealty by laymen were different from the oaths taken by clerics. Footnote Bernardus Parmensis, however, concluded that a vassal can swear again if he wishes, but he cannot be compelled to do so if he had sworn the oath according to the formula in Ego (Petrus) episcopus. Footnote

            What conclusions can we draw from this examination of the oath of fealty in the early jurisprudence of canon law? One recent study has concluded that the canonists were confused, bewildered, and clueless. They could not distinguish the ecclesiastical oath of fealty from the secular oath of fealty. They could not distinguish homage from fealty (as we have seen, in canon law they did not have to). The consequences of their confusion, it has been argued, would stretch far into the future. Footnote I have tried to demonstrate in this essay that the canonists distinguished very clearly between secular and ecclesiastical fealty. They also distinguished between fealty and homage. Whether they did so in the later Middle Ages will have to be studied in another place.

            Another major survey of feudal law has ignored canon law completely in describing the origins of the jurisprudence that shaped the secular oath of fealty and other aspects of feudal law. Footnote This historian did not think that the canonists were confused; she simply did not recognize the importance of canonistic jurisprudence on the development of feudal law. I have argued that the canonists were instrumental in developing the key norms governing secular oaths of fealty and applied them to a range of secular-oath-takers. They incorporated customary norms and mores into their thought. They drew upon Roman law and earlier canon law in their work. They did a good job of it. Their definitions of a vassal’s obligations would remain virtually unchanged for centuries, and traces of those obligations remain visible in modern civil law, especially in the doctrine of nonfeasance. They also applied many of the same norms that governed the secular oath of fealty to the ecclesiastical oath that prelates received from their clerics and that the pope received from bishops. In ax-large r context, this story is a good illustration of how the jurists of the Ius commune were not hampered by modern assumptions about the compartmentalization of legal systems. They did not hesitate to apply the norms that they found in the secular world to ecclesiastical institutions and to incorporate moral and ethical principles taken from religious law into the norms governing secular institutions. This cross-fertilization is one of the most important characteristics of the Ius commune and is primarily responsible for its influence on so many different aspects of modern jurisprudence.