How does soccer’s offside rule work?

There aren’t many moments as soul-crushing in soccer as when your favorite team has a goal stripped from the scoreboard when the linesman raises his or her flag to signal offside. The 2018 World Cup will feature several flags that squash rallies and erase momentary momentum thanks to an out-of-place player — but how exactly does the game’s offside rule work?

Offside will be one of the most common stoppages you’ll see in Russia, and the rule behind it is relatively simple. An offensive player without the ball can’t be closer to the opposing goal than the second-to-last defender on the pitch — typically the last man before the goalkeeper — at the time his teammate passes the ball to him.

Here’s a handy diagram:

The blue team is on offense in this picture, and the blue player marked “A” is in an offside position.

It’s a move that limits breakaway goals and prevents cherry picking; an attacking team has to remain more-or-less even with the last line of defense when moving without the ball. If you don’t have the ball, you have to either stay behind the ball or the next-to-last opposing player to remain onside.

A player is in an offside position if: he is nearer to his opponents’ goal line than both the ball and the second last opponent. A violation will occur when he is in an offside position at the same time the ball is being passed forward to him.

There are caveats to that rule. A player is NOT offside if:

That means even a single foot behind the defense’s second-to-last player is enough to trigger a linesman’s flag. That narrow margin of error will make things tough for officials in Russia, though this year, they will have video replay to help them out.

Need more information? Here’s FIFA’s official PowerPoint defining the rule.