reciprocal  (adj.): given or done in return; [grammar] expressing mutual action

We use reciprocal pronouns when each of two or more subjects is acting in the same way towards the other. For example, A is talking to B, and B is talking to A. So we say:

  • A and B are talking to each other.

The action is "reciprocated ". John talks to Mary and Mary talks to John. I give you a present and you give me a present. The dog bites the cat and the cat bites the dog.

There are only two reciprocal pronouns, and they are both two words:

  • each other
  • one another

When we use these reciprocal pronouns:

  • there must be two or more people, things or groups involved (so we cannot use reciprocal pronouns with I, you [singular], he/she/it)
  • they must be doing the same thing

Look at these examples:

  • John and Mary love each other.
  • Peter and David hate each other.
  • The ten prisoners were all blaming one another.
  • Both teams played hard against each other.
  • We gave each other gifts.
  • Why don't you believe each other?
  • They can't see each other.
  • The gangsters were fighting one another.
  • The boats were bumping against each other in the storm.
You probably noticed that each other is used in more examples above than one another. That's because in general we use one another (which sounds a little formal) less often than we use each other. Also, some people say that we should use one another only for three or more people or things, but there is no real justification for this.

sources : Original Link