Offside, without the guesswork.

How the call is made

1. Freeze the moment a teammate plays or touches the ball.
2. Check whether the attacker is in the opponents' half and closer to the goal line than both the ball and the second-last defender.
3. If not, play on. If yes, keep going because the player is only in an offside position so far.
4. Ask whether that player touches the ball, tries to play it, blocks an opponent, blocks the goalkeeper's line of sight, or clearly changes what an opponent can do.
5. If they do one of those things, it is offside. If they do not, there is no offence yet.

Details fans miss most

• Level is onside: if the attacker is exactly level, they are not offside.
• Position alone is not a foul: the player must become involved.
• Goalkeeper interference matters: a player can be offside without touching the ball if they block the keeper or stop the keeper seeing the shot.
• A deliberate play by a defender can reset the phase: but a rebound or save does not.

Only playable body parts count

An attacker is in an offside position only when part of the head, body, or feet is in the opponents' half and nearer the goal line than both the ball and the second-last opponent. Hands and arms are ignored for every player, including goalkeepers, with the arm boundary taken at the bottom of the armpit. Being level with the second-last opponent, level with the last two opponents, or behind the ball is onside. The snapshot is taken at the first point of contact when a teammate plays or touches the ball, except that a goalkeeper's throw uses the last point of contact.

Position turns into an offence through participation

An offside-positioned player is penalised for playing or touching the teammate's ball, challenging an opponent, clearly attempting to play a nearby ball when that action affects an opponent, obstructing an opponent's line of vision, or making an obvious action that clearly affects an opponent's ability to play it. Simply running toward the ball or standing beyond the defence is not automatically enough. Officials often delay the flag when a teammate from an onside position may collect the ball, then decide which player actually becomes involved.

Deliberate play resets; a deflection or save does not

An attacker receiving the ball from an opponent's deliberate play is normally not penalised for the earlier offside position, even when that deliberate play is inaccurate. The referee asks whether the defender had a realistic possibility of controlling the ball, considering its speed and distance, flight, time to coordinate movement, and whether the action was controlled rather than instinctive. A rebound, deflection, or deliberate save does not reset offside. A save means stopping or attempting to stop a ball going into or very close to goal, apart from using the hands illegally outside a goalkeeper's permitted area.

Three direct restarts cannot create offside

There is no offside offence when a player receives the ball directly from a goal kick, throw-in, or corner kick. Once another teammate touches the restarted ball, ordinary offside judgment begins from that new touch. When an offence is called, the defending team receives an indirect free kick from the place where the player became involved, even if that location is in the player's own half after returning from an earlier offside position. The restart is tied to involvement, not necessarily the spot where the original position was measured.

"He never touched it"

That does not end the discussion. If an offside attacker screens the goalkeeper, challenges for the ball, or forces a defender to react, the attacker may still be punished even without making contact.

Source material

• IFAB Law 11: Offside
• IFAB Laws of the Game
• FIFA Refereeing
• UEFA Refereeing