The outer ring of the dartboard is a double. So, when a player throws a dart into the dartboard’s outer ring, the points scored for that throw are doubled.
For example, if your dart lands in the double 20, that throw would be worth 40 points.
Checkout with a Double
In 01 darts games, you need to hit a double to effectively ‘checkout’ and win the game. For example, if a player has 40 points remaining, they must hit a double 20 with their last dart to check out and win the game.