This protocol is a standardized way of verifying whether a ball has landed in or out.
New since 2023:
Teams can challenge a referee's in/out decision up to a maximum of 5 seconds after the end of a rally. For this, any of the players shows a "C" to the 1st referee and then points to the line. Such a challenge is not possible at side changes.
A team is entitled to a maximum of two unsuccessful challenges per set.
If a challenge cannot be effective, e.g. because the ball has touched the line before landing out or in the case where a team has already exhausted two unsuccessful challenges during the running set, the challenge is rejected as illegal. In this case of a late challenge request (i.e. later than 5 seconds after the end of the rally or at side switch), the referee rejects the challenge request by showing an "X" with the index fingers.
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