Offside Call

Offside Call is the arcade's judgement game. There is no ball to kick and nothing to dodge. Instead you get a still image of the attacking third, seen from above, frozen at the precise instant the ball is played. Attackers are in amber, defenders in blue, the goalkeeper is in green, and a dashed amber line runs from the ball to the player who is about to receive it. That receiver is the one wearing the amber ring, and they are the only player whose position you are being asked about.

You then press one of two buttons. Onside waves play on; offside raises the flag. A bar across the bottom of the pitch is your clock — it starts at a little over four seconds and shrinks toward two as your run goes on, because the whole point of the exercise is that a real assistant referee does not get to think about it. Answer correctly and your count rises. Answer wrongly, or run out of time, and you lose one of your three lives.

The part that makes it worth playing rather than just guessing is what happens after every call. The game draws the line it actually used — a white dashed line across the pitch through the deepest outfield defender — and, when it matters, a second amber line through the ball. Right or wrong, you see the geometry that decided it. After twenty or thirty frames most players stop counting and start simply seeing the line, which is exactly what the drill is for.

1. Find the ringed player. Several attackers may be on screen. Only the one in the amber ring, at the end of the dashed pass line, is the player being judged. The others are there to make you look.
2. Find the line. Locate the defender in blue who is nearest to their own goal, ignoring the goalkeeper. That player's position is the offside line.
3. Compare, then check the ball. Is the ringed attacker closer to the goal than that defender? If not, it is onside and you are done. If they are, check the ball as well — a player cannot be offside if they are behind the ball.
4. Call it. Tap ONSIDE or OFFSIDE, or press ← for onside and → for offside. No call at all counts as a wrong one.
5. Read the replay. The lines appear for a moment before the next frame. That is the part that actually teaches you, so it is worth looking at even when you were right.

The law has three parts, and confusing them is why the rule has a reputation for being complicated when it really is not. A player is in an offside position if they are in the opponents' half and nearer to the opponents' goal line than both the ball and the second-last opponent. Being in an offside position is not an offence by itself. It becomes an offence only when that player becomes involved in active play — receiving the pass, interfering with an opponent, or gaining an advantage. And the position that matters is the one they occupied at the moment the ball was played by their team-mate, not the moment they received it.

The "second-last opponent" phrase confuses people, so here is the practical version. Opponents are counted from the goal line outward, and the goalkeeper is almost always the last one. That makes the second-last opponent the deepest outfield defender — the one this game draws its white line through. In the rare real situation where the keeper has come rushing out, the line falls on the second-deepest field player instead. This game keeps the keeper at home so the line is always the one you expect.

Level is onside

If the attacker is exactly level with that defender, they are onside. The law requires them to be nearer the goal line, and level is not nearer. The same applies to being level with the ball. This is the single most commonly misremembered part of the rule, and the game gives you frames that are dead level on purpose so that the habit gets built rather than argued about.

Behind the ball is onside

Both conditions have to be true at once: ahead of the defender and ahead of the ball. So when a striker has run clean through the defence and squares it backwards to a team-mate, that team-mate is onside no matter how deep in the box they are standing, because the ball is ahead of them. This is the case that catches nearly everyone the first few times, and it is why the game draws the amber ball line whenever the ball is beyond the defence.

No offside from a throw-in

There is no offside offence directly from a throw-in. There is also none from a corner or a goal kick. When a frame is a throw-in the game labels it clearly at the top of the pitch, and the answer is always onside however far beyond the defence the receiver is standing. It is a real rule that decides real games, and it appears from about the ninth frame onward.

• Scan in a fixed order every time. Deepest defender, then the ringed attacker, then the ball. A consistent routine is faster than a clever one, and it is the only thing that survives the clock dropping to two seconds.
• Do not count the goalkeeper. The keeper in green is the last opponent, so they are never the line. Looking at them costs you time and occasionally costs you the call.
• Check where the ball is before you flag. If the ball is deeper in the box than the receiver, the answer is onside and nothing else matters. One glance rules out a whole category of frames.
• Trust level. When it genuinely looks identical, the answer is onside. The margins in this game shrink to around two per cent of the pitch height, so "I cannot tell" is a meaningful signal rather than a failure.
• Read the throw-in badge. It sits at the top of the pitch and it is visible for the entire time you have to decide. It is not a trick — it is information you are expected to use.
• Ignore the decoys. The other amber players are frequently in blatantly offside positions. They are not receiving the pass, so their position is irrelevant. Only the ring matters.

Offside is hard for a reason that has nothing to do with knowing the law. An assistant referee stands level with the defensive line and has to watch two things that are metres apart — the ball being kicked and the attacker's feet — at the same instant. Human eyes cannot fixate on both, so the brain fills in one of them from a fraction of a second earlier or later. On a fast counter-attack, that fraction is enough to move a striker most of a metre.

There is also a well-documented perceptual bias called the flash-lag effect: a moving object appears slightly ahead of where it actually is at the moment something else flashes. It makes running attackers look further forward than they were, which is one reason marginal calls historically went against attackers more often than the geometry alone would predict. Video review removed the timing problem by letting officials stop the clock, and semi-automated systems removed the eye-position problem entirely — which is precisely why so many decisions now hinge on a distance smaller than a boot.

This game gives you the thing the real official does not have: the frame already frozen. What is left is the geometry, and the geometry is learnable. If you want the written version of the laws rather than the practical one, our Football Basics page walks through the pitch markings, the restarts and the rest of the rules in plain English. For a text-based test of the same knowledge, there is Football Quiz, and if you would rather be the one playing the through-ball, try Tiki-Taka Passing.

The two call buttons are drawn large along the bottom of the pitch, so a thumb reaches both comfortably on a phone in portrait. On desktop you can click them or use the arrow keys — ← for onside and → for offside — which most players find faster once the clock starts biting. The digits 1 and 2 work as well.

Every player position is stored as a fraction of the view rather than in fixed pixels, which means rotating your phone or resizing the window never moves anybody relative to anybody else and never changes the answer to the frame in front of you. Nothing is downloaded, there is no sign-up, and your best run is kept in your own browser. P pauses, and the speaker icon mutes the whistle and the ticking clock.