The search for the earliest manuscripts of the Vulgate text of Gratian’s Decretum can be aided by two textual variants that are important guides to deciding which manuscripts are the earliest versions of his text. Undoubtedly with more research others will be found. The first was discovered more than 25 years ago. Gratian had included a small section of Justinian’s Institutes in his Tractatus de legibus, D.12 c.6:
Diuturni mores consensu utentium approbati legem imitantur.
In the earliest manuscripts of the Vulgate, the text remained intact. Early on, however, the canonist interpolated the phrase, ‘nisi legi sunt adversi’, after ‘mores’. Brendan McManus examined this textual addition in a short essay in 1988. It has proven to be a secure guide to dating the earliest manuscript texts.
Kenneth Pennington, Gratian and Compurgation: An Interpolation, 31 BULL. MEDIEVAL CANON L. 253 (2014).