Like most things in football, the offside rule is pretty simple - but there are a few situations that can make the calmest of managers blow their top.
The 'active' ruling for example. Players can be in an offside position but not be offside. It doesn't quite make sense at first, but let's stick to the basics first.
A player is in an offside position if, when the ball is played by a team-mate, they are nearer to the opposition's goal line than both the ball and the second last opponent.
From the graphic above, the second last opponent determines the line from where the offside area begins and can be anywhere in this half of the pitch.
The referee's assistant will make their decision based on this offside area.
All straightforward so far? This is where the grey areas start to make life a little confusing.
A change to the rule was introduced two years ago, which allows a player to be in an offside position provided they are not "actively involved in play".
It was designed to promote attacking football, but it is open to interpretation.
Fifa, world football's governing body, gives the following definition:
"Interfering with play means playing or touching the ball passed or touched by a team-mate."However, a player does not necessarily have to touch the ball to influence play. They are still offside if they are judged to be:
Interfering with an opponent If an attacker interferes with an opponent by either preventing them from playing or being able to play the ball, then they are offside. For example, blocking the goalkeeper, or obstructing their line of vision.
Gaining an advantage If the ball is played into the penalty area and rebounds off a post, crossbar or an opposing defender, then the attacker is offside as they have gained an advantage by being in that position.
Offside Essentials
Here are a few more things to remember.
You can't be offside if:
* You receive the ball directly from a goal kick, a throw-in or a corner
* You are in your own half of the pitch
* You are level with the second last or last two opponents
* You are level with or behind the team-mate who plays you the ball
* You are not actively involved in play, as explained above
For any offside offence, the referee awards an indirect free-kick to the opposing team, to be taken from the place where the infringement occurred. 
