Quick History Lesson:
At the start of football, there were many rules. The original offside rule, Rule 6 of the Cambridge rules if I recall correctly, stated that any player in the opposition half is offside if played to him by a player on the same team. This resulted in players checking, which is when a player would form a /\ as the ones on the left/right would pick up possession if the one at the front was dispossessed. The Sheffield rules, saw this and changed it slightly stating that offside was deemed if it was in the opposing third. The Scottish rules were if 3 opposition players were between the goalline and the ball (1 more than the current day). It was then accepted into the rules of football later on with one less player.
:)

(Might have mixed up Sheffield/Scottish rules, but am like 75% sure Scottish is the more modern one)