Legacies and fideicommissa in general underwent a process of assimilation at the hands of Justinian, but there never was much real assimilation of direct and fideicommissary gifts of liberty. The former were a good deal relaxed in form, and this is so far an assimilation. But the fact that direct gifts could be made only to the testator's slaves and made them liberti orcini, while the others could be made to servi alieni and made them liberti of the fiduciarius, formed an unbridgeable gap between them. It must also be noted that a gift by fideicommissum is not in strictness manumission by will at all: it has to be completed by an act of manumission by the fiduciarius, and this will often be inter vivos. But as the direction is contained in a will or codicil, and, apart from condition, operates on entry, it is convenient to treat it here. Indeed more than convenience is involved: as we shall see later, a gift by fideicommissum is subject to the rules of the lex Fufia Caninia and to others specially applicable to testamentary provisions. On the other hand there is room in such gifts for causa, to complete a gift in some way defective, and this applies essentially to gifts inter vivos.

A fideicommissum of liberty is in effect a direction contained in a will or codicil, addressed to some person and requiring him to free a slave. It may even be in an unconfirmed codicil, and so bind the heres ab intestato. It may be given by word of mouth before witnesses, where the giver is in articulo mortis, and will then bind any heres.