Goal-line technology explained
What goal-line technology does, the problem it was created to solve, how it differs from VAR, and why it gives an instant, final answer.
What it decides
Goal-line technology answers one specific question: has the whole of the ball crossed the whole of the goal line? Under the laws of the game, a goal is only scored when the entire ball has passed completely over the line between the posts and under the bar. In fast, crowded goalmouth scrambles this can be almost impossible to judge with the naked eye, and goal-line technology exists to settle it beyond doubt.
The problem it solved
For years football was plagued by ghost goals: shots that clearly crossed the line but were not given, and balls wrongly judged to have gone in. High-profile mistakes in major matches, where officials simply could not see whether the ball had crossed, caused enormous controversy. The technology was introduced to remove these errors on the single most important decision in the sport, whether or not a goal has been scored.
How it works
Goal-line technology uses a system of cameras or sensors around the goal to track the ball's exact position. The moment the whole ball crosses the line, the system sends an instant, automatic signal, typically to a watch worn by the referee, confirming that a goal should be given. The decision is made in a fraction of a second and requires no stoppage, so the referee can award the goal immediately and with confidence.
Different from VAR
Goal-line technology is often confused with the video assistant referee, but they are separate systems. Goal-line technology does one job, deciding whether the ball crossed the line, and does it automatically and instantly. VAR is a wider review system, involving officials watching replays to check a range of decisions such as offside, penalties and red cards. The goal-line system's narrow focus is exactly why it can be so quick and certain.