What is the "Al-Umariyah issue" in inheritance?

The Two 'Umarite Cases' (al-'Umariyyatan) are two situations in inheritance where the mother inherits one-third of the remainder after the share of one of the spouses has been taken, and not one-third of the entire estate.

The first scenario: husband, mother, and father. The husband receives one-half. The mother receives one-third of the remaining half. The father receives the rest.

The second scenario: wife, mother, and father. The wife receives one-fourth. The mother receives one-third of the remainder after the wife's one-fourth. The father receives the rest.

They were named 'Umarite because 'Umar (may Allah be pleased with him) ruled in these cases with this judgment, and the Companions and Imams followed him, unlike Ibn 'Abbas who was of the opinion that the mother should receive one-third of the entire estate.