A rule of inference used in prepositional logic. Written 'Assoc.' for short. It allows you to change the parenthesis for order of operations in situations when it will make no logical difference, but will allow you to manipulate statements in useful ways. To demonstrate:

(P∨{Q∨R}) = ({P∨Q}∨R)

And

(P∧{Q∧R}) = ({P∧Q}∧R)

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