Ta'sib

The Islamic doctrine of residuary inheritance. After the Qur'anic heirs (ahl al-fara'id) receive their predetermined shares, any remaining balance is distributed among the residuary heirs (asaba) by ta'sib. Sons and other male agnatic descendants are the primary asaba; daughters can become asaba when they inherit alongside a son (taking half a son's share).

Three subtypes are recognized in classical fiqh: ta'sib through self (asaba bi nafsihi), as a derivative of another (asaba bi ghayrihi), and with another (asaba ma'a ghayrihi). Ta'sib is one of the three main inheritance-distribution doctrines, alongside radd and 'awl.

Ta'sib is the residuary mechanism of Islamic inheritance - the doctrine governing how asaba (residuary heirs) take what remains after fixed Qur'anic shares, and specifically how a male agnate converts related females into co-residuaries. Its signature operation: a son doesn't take a fixed share; he takes the residue, and his presence pulls his sisters out of their fixed-share status (a lone daughter's 1/2, two-plus daughters' 2/3) into sharing the residue with him at the Qur'anic 2:1 ratio (4:11 - 'for the male, the like of two females' portions').

The same conversion runs through other pairs: full brother with full sister, and the doctrine's priority ladder (sons and their line, then father, grandfather, brothers and their sons, then paternal uncles and theirs) determines who takes residue when no descendants exist. Understanding ta'sib resolves the most common confusions American Muslims bring to Islamic estate planning: why a daughter-only family sees part of the estate flow to the deceased's brothers (no ta'sib-triggering son exists, so daughters keep fixed shares and male collaterals take residue - unless radd or a different school's treatment applies); why shares cannot be pre-computed at drafting time (the residuary cast is unknowable until death); and why the 2:1 ratio exists at all (paired with men's nafaqah duty to financially maintain the family - the sister inherits half her brother's share but owes none of it to anyone, while his share arrives pre-encumbered with support obligations).

Faraid-compliant U.S. wills implement ta'sib by methodology reference, letting the executor calculate from the actual survivor set.