6.D.1. Players will call the lines on their side of the court (excluding the non-volley line on a serve - i.e., short serve, if being called by a referee).
6.D.5. No player should question an opponent’s call unless asked (except that player may appeal a call to the referee in an officiated match). A player should ask the opponent’s opinion if the opponent was in a better position to see the call. An opponent’s opinion, if requested, shall be accepted. The opinion of a player looking down the line is more likely to be accurate than one looking across the line.
The rule changes highlight the need for players to make fair line calls. In the spirit of keeping the game fun and avoiding conflict about line calls, here are some guidelines: