A testator may institute his heirs, if he pleases, in two or more degrees, as, for instance, in the following form: 'If A shall not be my heir, then let B be my heir'; and in this way he can make as many substitutions as he likes, naming in the last place one of his own slaves as necessary heir, in default of all others taking.

1 Several may be substituted in place of one, or one in place of several, or to each heir may be substituted a new and distinct person, or, finally, the instituted heirs may be substituted reciprocally in place of one another.

2 If heirs who are instituted in equal shares are reciprocally substituted to one another, and the shares which they are to have in the substitution are not specified, it is presumed (as was settled by a rescript of the Emperor Pius) that the testator intended them to take the same shares in the substitution as they took directly under the will.

3 If a third person is substituted to one heir who himself is substituted to his coheir, the Emperors Severus and Antoninus decided by rescript that this third person is entitled to the shares of both without distinction.

4 If a testator institutes another man's slave, supposing him to be an independent person, and substitutes Maevius in his place to meet the case of his not taking the inheritance, then, if the slave accepts by the order of his master, Maevius is entitled to a half. For, when applied to a person whom the testator knows to be in the power of another, the words 'if he shall not be my heir' are taken to mean 'if he shall neither be heir himself nor cause another to be heir'; but when applied to a person whom the testator supposes to be independent, they mean 'if he shall not acquire the inheritance either for himself, or for that person to whose power he shall subsequently become subject,' and this was decided by Tiberius Caesar in the case of his slave Parthenius.