301
A player must hit a "double" before any score is counted. See diagram 2 indicating the wires on the outer rim of the board. When a double is thrown, that dart will count, as will any following it.
A player's score is determined by subtracting from 301 the count of each dart. As shown on the diagram, the outer counts double; the smaller inner ring counts triple the designated score.
To win, a player must reduce his score to exactly zero, and the last dart must be a double. For example, if you need 41 to finish you could do it with a single 9 and a double 16.
When a player scores more than the exact score needed, his turn of three darts does not count; his score remains as it was before that turn. That means if 20 is required and the player scores 21, his score remains at 20. Also note that if the player hits a single 20, that turn does not count; his score remains at 20, and if 19 is hit, the throw does not count because you cannot go out on a single 1.