The offside law in rugby union may not be one that all fans know but it is one that is easy to learn.

Players must be onside to play legally in the sport. But some players do manage to get away with being offside at times.

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Some think this is just clever play, knowing what they can get away with. Opposition fans will just call them cheats.

In a nutshell, a player cannot get any advantage by being in front of the ball or in front of some of the imaginary offside lines on the pitch. A penalty is awarded against the transgressors.

What happens at the scrum?

At a scrum, maul or ruck, players not involved in the scrap for the ball have to be behind the back, or hindmost, foot of the last player. So, no going in at the side in a maul.

Also when a scrum is ongoing, the scrum-half must be behind the ball and if you are offside, you had better get back onside pronto before you even think about getting involved in play again.

If the ball is kicked from hand or from the kick-off, players have to be behind the kicker if they want to get involved in the chase or wait until the kicker has passed them.

The kick-off one is almost universally ignored by referees who let teams start their chase from in front of the kicker. All the hullabaloo over the Ken Owens‘s accidental offside in the final Lions Test against the All Blacks in 2017 would have been avoided if Kieran Read had been pinged for being in front at the restart.

Offside in rugby: What happens at the lineout?

At a lineout, the scrum-half and the thrower are allowed within five metres of the set-up with the defending hooker two metres from the line. The rest are banished back 10m and can only move for a long throw-in over the top or when the lineout ends and the mauling begins.

But straying offside can be costly. Clive Woodward still winces at the mere mention of 1981 and Cardiff. England were leading Wales 19-18, and in those days English wins at the Arms Park were rare, they had not won there for 18 years.

Of course the Welsh scrum-half Brynmor Williams dummied to pass, outlawed now, and Woodward rushed out of the defensive line to concede the match-winning penalty, check it out below. Steve Fenwick kicked the goal and England captain Bill Beaumont’s words to his centre are unprintable.

Accidental offside does occur when a player cannot avoid being touched by the ball or a team-mate who has got it in their mitts. If that player’s team get an advantage from that then it is a scrum. If there is no advantage the teams just get on with it.