Often shortened to 'AR'
Formerly known as 'Linesman'
By convention*, the AR runs along the right half of the touchline, facing the pitch from the side.
The AR is primarily responsible for signalling ball-outs
The AR has to indicate whether the ball is out* and which team put it out.
*Out means "All of the ball - over all of the line" viewed from above.
If the ball goes out over your touchline, then
Throw-ins
Correct Throws
For Foul Throws
Goal-Kicks
Corners
Standing in an offside position when the ball comes forward doesn’t mean they are offside. Don’t flag for offside unless they actually get the ball or they tackle a defender.
Other situations also count as ‘interfering with play’, eg, drawing a defender but these are a bit obscure for this basic page.
You need to run to stay level with the last defender in order to see if anyone is offside. Unless signalling, always keep your flag pointing down and pitchside so the ref can see it even when you are sprinting.
To indicate offside:
The offside rule (simplified):
Test yourself now with The Times Video Offside Test
The below duties are listed for completeness however be aware the most referees won't require a Club AR to perform any of these additional duties. Mostly their advice is "just stick to ball out of play and offsides, I'll look after the rest."
As soon as a sub is requested, hold your flag horizontal over your head with your arms straight to indicate to the ref that a sub has been requested. Hold it up until the the ref signals for the subs to come on.
If the ref doesn't want incoming player(s) to enter the field until the outgoing player(s) are off the ground, then call to the incoming subs "Wait!"
Keeping one eye on 22 players is an impossible task for the referee. The AR, therefore, acts as a second pair of eyes. It is their duty to indicate when an incident occurs out of the referee’s view by holding the flag straight up.
If the referee believes the linesman had a better view of an incident, they will sometimes come over and ask the AR what they saw. Only say what you are completely sure of, not what you think might have happened.
The AR moves to the goal line where the side of the penalty box meets it.
ARs rarely enter the field of play during the halves, remaining on the sideline for most of the game. If needed, an AR can enter the field of play to assist the referee during free-kick procedures, specifically to help enforce the 10 yards law.
ARs also help the referee when scuffles or fights occur. According to the FIFA Laws of the Game, the nearest assistant referee may enter the field of play to assist the referee in situations of mass confrontation.
The flag must always be unfurled and visible to the referee. This usually means carrying the flag in the hand closest to the referee and pointing down.
When making a signal, the assistant referee (AR) stops, faces the field of play, makes eye contact with the referee and raises the flag.
Especially don’t wave the flag about swatting flies unless you want the referee's attention.