F. Reciprocal Pronouns ‘each other’ and ‘one another’ when two or more people do the same thing.
a. Traditionally, each other refers to two people and one another refers to more than two people, but this distinction is disappearing in modern English. (e.g. Peter and Mary helped one another. or Peter helped Mary and Mary helped Peter.)
b. also use the ‘possessive forms’ each other’s and one another’s. (e.g. They helped to look after each other’s children. We often stayed in one another’s houses.) But we do not use reciprocal pronouns as the subject of a clause.