When can the chair umpire overrule a line umpire?

Under Appendix VII of the 2026 ITF Rules of Tennis, the chair umpire may overrule a line umpire or net umpire only when sure that a clear mistake has been made. The decided cases add:

• Immediately only: the overrule must come straight after the clear mistake. The chair umpire can't wait until the end of the point to overrule an earlier call in the rally.
• Never because of a protest: a chair umpire must never overrule as a result of a player's protest or appeal.
• Not when unsure: if the chair umpire couldn't see clearly but thought the ball was in, they may not overrule an "Out" call.
• If after an overrule the server is given a first service and the receiver argues it should be a second service, that is a question of tennis law, so the player may ask for the referee to decide.