In a standard 501 leg, a player must finish by hitting a double.
Players begin with 501 points and subtract the value of each dart. The final dart must reduce the remaining score to exactly zero, and it must land in a double ring or the inner bull, which counts as double 25. This rule is commonly called double-out.
For example, a score of 32 can be completed with double 16. A remaining score of 50 can be finished with the inner bull. A player who reduces the score below zero, reaches one, or reaches zero without a double usually busts and returns to the score held before that visit.
Some recreational formats use single-out or other finishing rules, so the phrase standard 501 normally refers to double-out play. The World Grand Prix adds an additional double-start requirement.