Offside Law -
Law 11 -
A player is in an offside position if he is nearer to his opponent's goal line than both the ball and the second last opponent.
The occasions when a player is not in an offside position are largely clearly understood:
Nearer to his opponents' goal line or level with the second last or last opponent.
"Carry the centre line principle when judging."
This also causes incorrect challenges to referees' decisions by participants and spectators who are concentrating on the ball and then make their judgement when the player receives the ball. Whilst the ball is moving forwards the defender moves out and the attacker moves forward and sometimes the gap between them is enormous but the player was not in an offside position when the ball was released.