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How is it possible for the assistant referee to realize when a player is offside or not? If the assistant referee is in line with the last forward how can he see when the ball leaves the other player if he is so far from the last forward? Watching the replays, it seems impossible to make the right decision on difficult offside calls.

The problem is that is necessary to view at the same time tha last players (forward and defender) and the ball.

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3 Answers 3

up vote 13 down vote accepted

When the ball is in play, the assistant referee will do their best to remain in line with whichever one of

  • the second closest defending player to the goal line;
  • the ball; or
  • the halfway line

is closest to the goal line (p.86 - FIFA 2014/15 Laws of The Game).

They will generally side-step, only running facing the goal line when required to do so to keep up (p.86, p. 95 - FIFA 2014/15 Laws of The Game). This enables them to have a view of the offside line, the ball, the referee and the majority of the players at once. By occasionally placing their chin on their chest, they can use the touch line to gauge whether they are remaining square with the offside line.

When it appears that an attacker is going to play a ball and other attackers are close to being offside, the assistant referee will focus on the offside line and use their peripheral vision to judge when the ball is last played.

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Solid answer, a source or two would be awesome –  wax eagle Jul 16 at 13:20
    
@waxeagle - sources added for the parts that aren't backed just by personal refereeing experience. Please let me know if there's anything else I can improve. –  studro Jul 16 at 13:26
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It should also be added that, apart from in the top divisions, the assistant referee can also use his/her hearing as a compliment to peripheral vision. The sound of a boot hitting the ball is often quite recognizable, so it's often useful to listen - at least I did so when I was active as an assistant ref. +1 for a great answer! –  Qvist Jul 16 at 14:09
    
I was tempted to add it, as it may be a useful technique (not one that I've tried personally), but I started crunching the numbers. Using the sound is only a good idea for calls that aren't particularly close, or in games where the players aren't fast. Sound travels roughly 340 metres in a second. As an assistant referee, on average, you'll be about a tenth of that distance away, and potentially up to double it. Players in the modern game can approach 10m/s at top speed. So on average, your calls will be out by one metre, and perhaps as much as two metres. –  studro Jul 18 at 3:42
    
However, in spite of that, I do think the sound of boot/knee/body/etc on ball is very good for making decisions based on who touched it last. –  studro Jul 18 at 3:44

This is what makes it such a difficult job, and why I can't understand why sometimes people are so harsh on linesmen when the decision is tight.

The linesman's optimal place is in-line with the second last defending player. From here he will be able to judge the hardest part of his decision, the placement of the attacker.

He will then have to use his best judgement from peripheral vision and other factors as to when the ball was passed.

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"Last defender" isn't entirely correct. –  studro Jul 16 at 12:30
    
A source would be super helpful here. –  wax eagle Jul 16 at 12:42
    
Adding to studro's comment, it's most probably "2nd to last defender" you are referring to. The last defender is the goalie (usually). –  nhahtdh Jul 16 at 13:17
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Yeah, I'll edit for that correction. By defender I meant outfield defending player. Which in most cases is correct, but obviously not always. –  Tom.Bowen89 Jul 16 at 15:14
    

Further to previous, the assistant referee remains level with the second rearmost defender for the majority of the time. He/she will only leave this position if the ball is further forward than the second last defender ie. the ball has travelled beyond the defence/attack and is going to the goalkeeper. They do this in case the ball travels over the goalline and therefore they need to be level to see it has crossed completely over the line. However, as I say for the majority of the time they will stay level with the second rearmost defender. This is because this is the optimum position for judging offsides.

If the assistant is level with the second rearmost and an attacking player is to the right of them, then this attacking player is in an offside position. Then dependant on the other criteria of Law 11, the assistant will decide whether to raise the flag and indicate offside. It is important to understand that it is not an offence to be in an offside position. It only becomes an offence when they recieve the ball or interfere with an opponent etc. There will be numerous times during a game when the assistant will 'keep the flag down' allowing play to continue because even though the player is offside, an offence is not being committed

As for how and when to judge offside, studro makes a great point that the body position of the assistant is so important. By being square on to the field of play they have a great view of when the ball is played and also the line of players in front of him. Very little head movement is required as it where.
In England, this movement is known as crabbing and there is a great emphasis on this. Assistant Referees physical training plans usually include some of this sidestep running. Even being a metre off being level with the second rearmost defender can cause an error in offside judgment. This means when looking across the field of play you do not have a true view of whether a player is offside

The difficulty comes when the ball is on the same side as the assistant just a bit further down the line from him. The assistant is then required to turn his head left to watch the player about to kick the ball and also across the field to study the offside line. Add into this the assistant is watching for any fouling, and the ball crossing the line for a throw in, even a quick glance away from across the field of play (ie. away from the second last defender) can cause you to suddenly be not level any more. As per previous responses, listening for the ball being kicked is useful and it is surprising that even in grounds with many thousands of people you can still hear the thud of the kick. A trick used in the english game is for the other assistant or referee to help the assistant by using the communication kit and tell him that the ball has been played forward. This means the assistant can keep watching the line and know the ball has been played forward. Additionally the assistant will do similar on a throw in deep in the referees half. Rather than have the referee watch the throw (and take his eyes away from the players), he can watch the players and the assistant will say "In play". This means should the referee notice any pushing, if he blows his whistle, he will know if the ball was in play or not. If the ball was in play he would need to award a freekick. If not in play then he could say "calm down fellas" before allowing the throw in.

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