Cologne, Dombibliothek 127, fol. 9r

Cologne, Dombibliothek 128, fol. 10v

Gratian, Decretum, D.1  c.5

Custom (consuetudo) is a certain kind of ius established by mos and received as lex  when there is no lex.  It does not matter whether it is confirmed by writing or by reason, since reason also supports all leges.  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.  Custom (consuetudo) is so called because it is in common use.

Gratian:  So, when it says, "it does not matter whether custom (consuetudo) is confirmed by writing or by reason," this shows that, in part, custom (consuetudo) has been collected in writing, and, in part, it is preserved only in the usages of its followers. Those things that are put into writing are called constitutions or Ius;  those things that are not put into writing, are called general name, namely custom (consuetudo).

[Isidor. 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.
<Gratianus>: Cum itaque dicitur: "non differt, utrum consuetudo scriptura, uel ratione consistat," apparet, quod consuetudo partim est redacta in scriptis, partim moribus tantum utentium est reseruata. Quae in scriptis redacta est, constitutio siue ius uocatur; quae uero in scriptis redacta non est, generali nomine, consuetudo uidelicet, appellatur.