A reciprocal pronoun is used when two or more people do the same action to each other. If Sarah helps Daniel and Daniel helps Sarah, the action goes both ways — and English marks that with a single short form.
English has only two reciprocal pronouns: each other and one another. Both are written as two separate words.
When you can use them
A reciprocal pronoun needs a plural subject — at least two people, or a group. One person alone cannot reciprocate anything.
You cannot say I love each other or She called one another. The subject must contain more than one participant, either as a compound (Anna and James) or as a plural noun (the team members, the twins, the neighbours).
Reciprocal pronouns are never the subject of the sentence. They sit as the object of a verb or after a preposition.
Each other vs one another
In modern English, each other and one another mean the same thing and can be swapped freely. Each other is far more common in everyday speech and writing; one another sounds slightly more formal.
Some older style guides claim that each other is only for two people and one another only for three or more. This rule has existed for about 300 years and has been ignored for almost as long — good writers have used both forms interchangeably since the sixteenth century. Treat it as a historical note, not a binding rule.
The possessive form
To show possession, add an apostrophe and s: each other's and one another's. The apostrophe always goes before the s, never after.
The form is always singular — each other's, not each others'. Even though the pronoun refers to two or more people, grammatically it behaves like a singular word when it takes an apostrophe.
Reciprocal vs reflexive
Reciprocal and reflexive pronouns look related, but they point in different directions. A reflexive pronoun turns the action back onto the subject itself. A reciprocal pronoun sends the action between members of the group.
The contrast is small but sharp. If the action stays inside each individual, use themselves. If it travels from one person to another and back, use each other or one another.
Some verbs already carry a reciprocal meaning — meet, argue, hug, kiss. With these, the reciprocal pronoun is often optional: They met and They met each other say almost the same thing. Reflexive forms are covered separately.
Common mistakes
Subject must be plural
Position in the sentence
Written as one word
Possessive form
Confusion with reflexive