How is the number of shares calculated in inheritance cases?

In the science of inheritance (ilm al-mawarith), calculation involves establishing the basis of issues (ta'seel al-masaa'il) and correcting them (tashih al-masaa'il). Ta'seel is the determination of the smallest number from which the prescribed shares of the issue can be derived. Tashih is the extraction of the smallest number that allows each heir's share to be obtained without a fraction.

Thus, if a person passes away leaving a wife and a son, the basis of the issue would be eight shares: one share for the wife (one-eighth), and seven shares for the son (the remainder by agnation).

The agreed-upon bases for issues are seven: (2, 3, 4, 6, 8, 12, 24). Some scholars have added two more bases in the chapter concerning the grandfather and brothers: (18, 36).