Offside is a complicated part of rugby union.
Different phases of the game have their own set of offside laws.
OFFSIDE IN OPEN PLAY
If a player is in front of a team-mate in possession of the ball, or in front of a team-mate who last played the ball, they will be offside if they:
Actively try to play the ball
Do not retreat within 10m of an opponent who is waiting for the ball
Move towards the opponents or the place where the ball lands without first coming back onside
The referee will award a penalty at the place where the offence took place.
OFFSIDE AT A SCRUM
For scrum-halves, the offside line is the line of the ball fed into the scrum.
That means they can't go beyond that line until the ball has been put into the scrum by the opposing number nine.
For all the other players, the offside line is an imaginary line drawn through the 'hindmost' foot of the last player in the scrum.
No player apart from the eight forwards and scrum-halves are allowed within this area.
The opposing scrum-half has to wait until the ball is out of the scrum before making a tackle for the ball.
If they don't, the referee will award a penalty.
OFFSIDE AT A RUCK OR MAUL
Most offside decisions in rugby union happen at rucks and mauls, especially when the ball is being recycled a lot.
Like the scrum, an imaginary line is drawn through the hindmost foot of the last player in the ruck or maul.
Players must either join the ruck or maul or retreat behind the offside line.
A player is offside if they:
Join from their opponent's side
Join play from in front of the last man
Do not join either the ruck or maul, but fail to get behind the offside line
Leave the ruck or maul, but do not get behind the offside line.
OFFSIDE AT A LINE-OUT
There are two different imaginary offside lines for players involved in the line-out and those who are not.
Effectively they make a box which is 10m wide either side of the line-out.
Only the forwards and the scrum-half are allowed in this area until the ball has been thrown in, touched a player or the ground.
The referee will create a one-metre gap between the two lines of forwards. This is the offside line between the two sets of forwards.
Players must not encroach within this gap until the ball has touched a player or the ground, unless they are jumping for the ball.
No player can leave the line-out until it has finished.
OFFSIDE AT A KICK
If a player is about to kick a high up-and-under or a grubber kick for a team-mate to run onto, the chasing player must be level or just behind the kicker.
If they are not, the referee will award the opposition a penalty.
If the player is in front of the kicker already, they cannot get involved with open play.
Players often raise their arms when running back to an onside position.
This shows the referee that they have no intention of joining play because they are in an offside position.