There are many people wondering what measure is used to determine whether it is offside when playing football. There is still a problem in understanding the offside rule. You may also ask why players try to make such mistake; however, being offside is of two types: being in an offside position, and committing an offside offence.
To be in an offside position is to be closer to the opponent’s goal than the last opposition outfield player (except the goalkeeper) and the ball. Yet, when a player commits an offside offence he will have the ball played forward towards another player who is already in the position considered offside.
The rule is further complicated by the fact the referee or his assistant must adjudge you to be ‘active’ in the play before giving an offside decision against you. This can be obvious, for example if you touch the ball in an offside position, but it can be extremely nebulous. The official rule states ‘active’ as meaning “interfering with play… or an opponent… or gaining an advantage by being in that position”. However, as you will find as you watch more and more games, what one referee or linesman considers to be active can be very different to another individual’s interpretation, and the offside rule is generally a major debating point.
There are other factors to consider which can exempt you from the offside rule. You cannot be offside in your own-half of the pitch, for example, and you cannot be penalised for being in an offside position when a goal kick, throw-in, indirect free kick or corner kick is taken.
Picture and most part of the explanation: www.talkfootball.co.uk