The firstborn is entitled to a "double portion," that is, he takes twice the portion due to each of his brothers from their father's inheritance. Thus if the father has left a firstborn and two other sons, the former takes one-half and the latter one-quarter each of the estate (BB 122b–123a and Posekim). The prerogative does not extend to the mother's estate (BB 111b, 122b and Codes).
The firstborn takes a double portion only of the present and not of the contingent assets, i.e., only of the assets in the father's possession at the time of his death and not such as were due to come into his possession thereafter. Thus, if the father predeceased any of his own legators, the father's share in their estate passes through him to his own heirs, the firstborn taking only the share of an ordinary heir. This rule embraces debt still owing to the father at his death, even if under deed or bond, since the debt is considered an asset still to fall due and not yet in possession. If, however, the loan was secured by a pledge, or mortgage, the firstborn takes a double portion since in Jewish law the creditor acquires a right over the pledged property (Git. 37a) and a loan thus secured is therefore considered as an asset in possession (see generally Bek. 51b–52a; BB 125b; commentators and Posekim ad loc.). For the same reasons the firstborn does not take a double portion of improvements or increments from which the father's estate has benefited after his death, except with regard to natural increments – as for instance in the case of a sapling which has become full-grown (ibid.).