The first test case is Causa 24. The purpose of this chapter is to demonstrate that a close reading of this causa supports the thesis that the text of the manuscripts Aa and Fd is a first recension of Gratian's Decretum (the incomplete Bc P and Pfr do not contain C. 24). This demonstration follows two lines, showing that the text of Aa and Fd presents a coherent and complete argument, and that the two recensions used different sets of sources.

Causa 24 is the second of the causae hereticorum, so called since the author of the second recension at C. 7, q. I, d. p. c. 48 refers to C. 23 with the words “in prima causa hereticorum.” Both Causae 23 and 24 are concerned with the treatment of heretics and the latter with the mechanics of excommunication (and reconciliation) in particular. In the second recension, C. 24 contains eighty-eight canons (and one palea), thirty-nine of which are (wholly or, in three cases, in part) present already in the first recension. The causa is, thus, of a reasonable size in the first recension and there are substantial additions in the second recension. Both circumstances make it a suitable test case.