[1] However, the courts may invalidate side letters in conflict with the main collective bargaining agreement.
In some cases, side letters have driven national labour law policy.
For example, in the United States, a side letter guaranteeing employer neutrality during union elections in newly-acquired plants, subsidiaries, or divisions led to a federal lawsuit over the legality of the agreement in 2002 and to a major decision by the National Labor Relations Board that revised federal labour policy in 2007.
[5][6] In Australia, side letters are becoming increasingly common due to the changes in federal labour law that the WorkChoices Act created.
The inclusion of even minor non-workplace-related clauses (such as dues check-off) can render a CBA unenforceable.