We use the reciprocal pronouns each other and one another when two or more people do the same thing.
Peter and Mary helped each other.
= Peter helped Mary and Mary helped Peter.
We sent one another Christmas cards.
= We sent them a Christmas card and they sent us a Christmas card.
They didn't look at each other.
= X didn't look at Y and Y didn't look at X.
We also use the possessive forms each other's and one another's:
They helped to look after each other's children.
The group of students often stayed in one another's houses.
Note that we do not use reciprocal pronouns as the subject of a clause.
- Reciprocal pronouns 1
Reciprocal pronouns and plural reflexive pronouns (ourselves, yourselves, themselves) have different meanings:
- Reciprocal pronouns 2