Institutes (institutiones). This was one of the titles given to elementary textbooks of Roman law. The best-known work of this kind is the Institutes of *Gaius(2). This was taken by *Justinian as the basis of his own Institutes. Though intended, like its model, as a students' manual, this work was given legislative validity. It was compiled by *Tribonianus and the professors Theophilus and Dorotheus, who had also been among Tribonianus' collaborators in the compilation of the Digest (see justinian's codification). It is essentially a cento or patchwork. The structure and a substantial part of the content come from the Institutes of Gaius, but the rest is taken from other classical elementary works, together with matter supplied by the compilers themselves to deal with post-classical changes in the law (mainly those made by Justinian himself). The principal classical sources (apart from Gaius' Institutes) are the Res cottidianae, another elementary work attributed to Gaius, and the Institutes of other jurists (Florentinus, *Ulpian, *Marcianus).