Ball in and out of play explained
When the ball is in or out of play, why the whole ball must cross the whole line, what happens off the posts and officials, and how play restarts.
The basic rule
The ball is in play at almost all times during a match, including when it rebounds off a goalpost, crossbar or corner flag and stays on the field. It goes out of play in only two situations: when it has wholly crossed a boundary line, either the touchline or the goal line, whether on the ground or in the air, or when the referee stops play. Until one of those happens, the game simply continues.
The whole ball, the whole line
The key detail is that the entire ball must completely cross the entire line for it to be out of play. If any part of the ball is still touching the line, it is still in. This is the same principle used to judge a goal, where the whole ball must cross the goal line between the posts. It means a ball can look as though it has gone out while, technically, part of it is still overhanging the line and in play.
Rebounds and the referee
Because the boundary is the line itself, the ball staying on the field after striking a post or the crossbar is still live, and quick players will follow up such rebounds. The ball is also still in play if it strikes the referee or an assistant and stays on the pitch, though play may be stopped in certain situations, such as if a promising attack results. Otherwise the game carries on as normal.
Restarting play
How the game restarts depends on where the ball went out and who touched it last. If it crosses the touchline, a throw-in is awarded to the team that did not touch it last. If it crosses the goal line without a goal, it is a corner if an attacker was last to touch it and a goal kick if a defender was. Knowing exactly when the ball is out is what determines every one of these restarts.