Pickleball gives players responsibility for most line calls on their own end of the court. The rule is not "close balls are out." It is the opposite: if the ball cannot be called out with certainty, it is in.
During a rally, a ball is in if it lands on the opponent's end of the court. That includes a ball touching a sideline or baseline. A ball is out only when it lands outside the court.
The practical test is certainty. Players should not call a ball out unless they clearly see space between the ball and the line when it lands. If the view is blocked, the bounce is too fast to judge, or partners are unsure, the ball should be considered in.
A served ball must travel diagonally and land in the correct service court after clearing the non-volley zone. A serve that lands on the non-volley-zone line is short and is a fault.
That exception does not make every service-court line out. A serve that clears the kitchen line and lands on the correct sideline, centerline, or baseline is treated as in, assuming no other service fault occurred.
In ordinary non-officiated play, players make line calls for balls landing on their own end. In doubles, either partner may make the call, but a disagreement between partners means the team's call is in.
A player may ask the opponent for an opinion before the next serve. If the opponent gives a clear answer, that answer stands. If the opponent cannot make a definite call and the original team made no valid out call, the ball is in.
An out call after the ball lands is a line call, and a valid line call stops the rally. If a player returns the ball and also wants to call it out, the call must come before the opponent hits the next shot or before the ball otherwise becomes dead.
Partner talk while the ball is still in the air, such as "watch it" or "might be out," is communication, not a line call. The actual line call happens after the ball lands.
A line call decides whether a ball landed in or out. A fault is a rule violation that ends the rally, such as volleying before the required bounce, hitting the ball out of bounds, being hit by the ball after the serve, committing a service foot fault, or volleying from the non-volley zone.
Players are expected to call faults on themselves or their partner. In non-officiated play, players may call only non-volley-zone faults and service foot faults on an opponent. For most other opponent faults, they may mention the issue after the rally, but they do not have authority to enforce it.
In an officiated match without line judges, players still make many line calls on their own end, and a referee may decide an appealed rally-ending shot only if the referee clearly saw the landing spot.
If a referee overrules a player's out call because the ball landed in, the player or team that made the out call has committed a fault. If a referee cannot make a definitive call, the result depends on whether there was an original player call, an opponent opinion, line judges, or no call at all.