An irregular noun is one that for some reason does not follow the regular pattern and thus a masculine noun which takes feminine plural endings or a feminine noun which takes masculine plural endings are considered irregular nouns. Generally there is no way of knowing how these nouns will respond and thus they must be memorized. Common Irregular Nouns: Meaning “father” “brother” “sister” Sing. abs. אַב “father” אַח “brother” אָחוֹת “sister” cons. אַב אֲבִי “father of”