1 A necessary heir is a slave of the testator, whom he institutes as heir: and he is so named because, willing or unwilling, and without any alternative, he becomes free and necessary heir immediately on the testator's decease. For when a man's affairs are embarrassed, it is common for one of his slaves to be instituted in his will, either in the first place, or as a substitute in the second or any later place, so that, if the creditors are not paid in full, the heir may be insolvent rather than the testator, and his property, rather than the testator's, may be sold by the creditors and divided among them. To balance this disadvantage he has this advantage, that his acquisitions after the testator's decease are for his own sole benefit; and although the estate of the deceased is insufficient to pay the creditors in full, the heir's subsequent acquisitions are never on that account liable to a second sale. 2 Heirs who are both family heirs and necessary are such as a son or a daughter, a grandchild by a son, and further similar lineal descendants, provided that they are in the ancestor's power at the time of his decease. To make a grandson or granddaughter a family heir it is, however, not sufficient for them to be in the grandfather's power at the moment of his decease: it is further requisite that their own father shall, in the lifetime of the grandfather, have ceased to be the family heir himself, whether by death or by any other mode of release from power: for by this event the grandson and granddaughter succeed to the place of their father. They are called family heirs, because they are heirs of the house, and even in the lifetime of the parent are to a certain extent deemed owners of the inheritance: wherefore in intestacy the first right of succession belongs to the children. They are called necessary heirs because they have no alternative, but, willing or unwilling, both where there is a will and where there is not, they become heirs. The praetor, however, permits them, if they wish, to abstain from the inheritance, and leave the parent to become insolvent rather than themselves. 3 Those who are not subject to the testator's power are called external heirs. Thus children of ours who are not in our power, if instituted heirs by us, are deemed external heirs; and children instituted by their mother belong to this class, because women never have children in their power. Slaves instituted heirs by their masters, and manumitted subsequently to the execution of the will, belong to the same class.