The practical informality and flexibility of fideicommissum is described in Institutes, 2.23.2. There was no need for a certain formula, any word describing the beneficiaries, such as rogo, peto, or volo, employed with the term "fidei tuae committo" sufficiently instituted a fideicommissum.Kaser 6, § 77 II 1. It could be constituted in a will or in a codicil, made orally or even declared by mere gestures. Most attractive of all, it could be added or revoked or varied after the institutio heredes itself.As shown in G. 2, 249; PS 4, 1, 5-6; and Buckland/McNair p. 163; 170; supported by Borkowski/du Pellis 8.9.2.3. Taking all these advantages in account, it is not surprising that the legatus, with its strict formal requirements, was far less popular.The terms under which the legatus was instituted are explained in Buckland, Main Inst. p. 231, with details on the gradual assimilation of legates and fideicommissa, which under Justinian led to the fusion of the two former separate institutions (an example of the vulgarisation of Roman law after its classic era). In later eras, a gradual assimilation of legates and fideicommissa took place; under Justinian, the two institutions were fused, in an example of the vulgarisation of Roman Law after its classic era.
