Novice fans don’t understand it. Longtime fans claim to understand it, but then openly disagree about it. Referees and their assistants are trained to spot it, but often have to turn to replays to make sure they’ve got it right.
But now you will understand it.
A Classic Example
As a rule of thumb, the key thing to look for is when only the goalkeeper is between the player and the goal when the ball is passed. That’s offside.
In this case, the attacking player is onside when the ball is passed, and so is not offside, even though the player then moves offside before receiving the ball.
A Close Call
If any part of a player’s body that can legally score a goal is past the last defender — a foot, a head, a knee, even a backside; basically anything other than the lower arm — the player is offside.
An Offside Teammate
The officials must judge if the player without the ball is involved in the play in some way — for example, by challenging for the ball or, say, obstructing the goalkeeper’s vision. In that case, the player would be judged offside despite not having touched the ball.
An Acceptable Position
In this case, the attacking player uninvolved in the action is far enough away to be considered what is called passively offside. As long as this player stays out of the play, the red team can continue the attack.
This player looks offside. But the play is a corner kick. If a player receives the ball directly from a corner kick, that player cannot be offside. The same is true for throw-ins and goal kicks.
This player seems to be offside. But the play is taking place in the team’s own half, and a player cannot be offside in the team’s half of the field.
And that’s it. Now, grab a flag — you’re fully qualified to be a referee’s assistant at the World Cup. Well, nearly.