Köln, Erzbischöfliche Diözesan- und Dombibliothek  127, fol. 9rb (ca. 1160 A.D.)

 

 Gratian, D.1  c.5: Isidore of Seville, Etymologies Book 5 c.3

Consuetudo is a sort of ius established by mos and recognized as lex when lex is lacking.  It does not matter whether it is confirmed by writing or by reason, since reason also supports lex.  Furthermore, if lex is determined by reason, then lex will be all that reason has already confirmed --- all, at least, that is congruent with religion, consistent with discipline, and helpful for salvation.  Consuetudo is so called because it is in common use.
[Isidore  eod. cap. 3. et lib. II. c. 10.]
Consuetudo autem est ius quoddam moribus institutum, quod pro lege suscipitur, cum deficit lex. Nec differt, an scriptura, an ratione consistat, quoniam et legem ratio commendat. Porro si ratione lex constat, lex erit omne, iam quod ratione constiterit, dumtaxat quod religioni congruat, quod disciplinae conueniat, quod saluti proficiat. Vocatur autem consuetudo, quia in communi est usu.