Reciprocal Pronouns

We use the reciprocal pronouns each other and one another when two or more people do the same thing. Subject

each other

Peter and Mary helped each other
Peter helped Mary and Mary helped Peter.

one another

We sent one another cards.
We sent them a card and they sent us a card.

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 and plural reflexive pronouns
ourselves • yourselves • themselves have different meanings:

John and Fred talk to each other regularly.
John talks to Fred and Fred talks to John.

John and Fred regularly talk to themselves.
John talks to himself and Fred talks to himself.