It's confused football fans for years, but now the offside rule has been explained on the back of a 50p coin.
The Royal Mint has issued the coin as part of a commemorative series to celebrate the 2012 Olympics.
One coin has been issued for each of the 29 sports to be competed at the Games this summer, with the designs chosen after a nationwide competition.
You can see all the coins right here with this superb slideshow put together by our colleagues at Yahoo! Finance.
But it is the football coin which has drawn the most attention, with its brilliantly simple illustration explaining the offside rule with just a few easily-understood symbols.
The design was the brainchild of Neil Wolfson, a production journalist from London, who had the bright idea of illustrating the beautiful game not with a stylised action picture but with a reference to one of the sport's perennial talking points.
Other highlights from the collection include the beautifully-dramatic canoeing and modern pentathlon coins, and the cartoon high-jumper on the athletics coin designed by nine-year-old Florence Harper from Bristol.
All the coins can be bought direct from the Royal Mint for £2.99 each, not great value compared to the face value of the contents, but not bad value way for those who don't want to take their chances waiting for a particular coin to turn up in their change.