reciprocal pronoun

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We have only two of these pronouns, and they come in pairs: each other and one another. We use them to show some sort of a relationship between two or more people or larger groups.

In formal settings you should use each other to refer to two people or two groups; one another, to three or more. In less formal settings, we use the terms interchangeably.

The reciprocals have possessive forms. Each of these is singular, not plural, as in Susan and Tom criticized each other's political views.

Here are some examples:
Tom and Fred like each other. All the class members dislike one another.