What is the difference between 'each other' and 'one another'?

Both 'each other' and 'one another' are reciprocal pronouns, showing that two or more people do the same thing to each other. The traditional rule is to use 'each other' when talking about two people or things, and 'one another' when talking about more than two. For example, 'The two brothers helped each other' (two people), but 'All the team members supported one another' (more than two). In everyday modern English the two are often used interchangeably, and most examiners accept either. But if your textbook teaches the strict rule, follow it: two means 'each other', three or more means 'one another'. Remember both stay singular as a phrase and take an apostrophe only when showing possession, like 'each other's books'.