How an offside decision is built, from the phase to the line

Position is not an offence

Standing in an offside position is not itself an infringement, which is the single most widely misunderstood feature of the law. The offence arises only when a player in that position becomes involved in active play, whether by interfering with an opponent, with play, or by gaining an advantage. That is why a player can stand well beyond the defensive line for long stretches without any flag being raised against him.

The distinction shifts the question from where a player was to what he then did, which is a considerably harder thing to judge at speed. Most contested decisions involve interference with an opponent, where the assessment concerns sight lines and challenges rather than distances at all.

The moment the ball is played

Position is assessed at the instant the ball is played by a teammate, not when it arrives and not when the run was begun. Identifying that instant is a genuine problem, because the foot is in contact with the ball across a brief period rather than at a single point in time. A deflection off a defender does not restart the phase, whereas a deliberate play of the ball by a defender generally does.

The difference between a deflection and a deliberate play is therefore load-bearing, and it turns on control rather than on contact. This is where a decision that looks like a measurement actually rests on a judgement made before any line has been drawn.

Which part of the body counts

Only parts of the body with which a player may legally play the ball are considered, so arms and hands are excluded from the comparison entirely. The forward-most such part of the attacker is compared with the equivalent part of the second-last defender at the chosen instant. Defining where the arm ends is unavoidably approximate, which is one reason very fine decisions attract disagreement even with clear footage available.

The goalkeeper is usually but not necessarily one of the two defenders, since the law refers to the second-last opponent rather than to any named position. A defender who has left the field in the course of a movement is still counted, which prevents a defence manufacturing an offside by stepping off the pitch.

Where the law does not apply

There is no offside from a throw-in, a goal kick or a corner kick, and those exceptions are older than most of the surrounding law. A player in his own half cannot be offside, which is why deep-lying attackers move freely while the ball is worked forward from the back. A player level with the second-last defender is onside, so the comparison is strict rather than treating a tie as an infringement.

These exceptions define the situations in which a defence cannot rely on stepping up, and they shape how set pieces have to be defended. A team defending a corner must mark rather than hold a line, which is a different discipline requiring a different kind of concentration.

Technology and the argument it did not settle

Automated systems track body points and derive the comparison mechanically, which removes the human difficulty of observing two things in the same instant. What they cannot remove is the definitional work upstream, since the system still has to be told when the ball was played and which point on the body counts. The visual output looks like a measurement, and that appearance is why marginal calls now generate more argument rather than less.

Some competitions have responded by introducing a tolerance, effectively declining to act on differences smaller than the instrument can meaningfully resolve. That is the same reasoning behind margin provisions in other sports, and it is a statement about the instrument rather than about fairness.