The idea behind the wire numbers is to spread the wear so the dartboard lasts longer. The treble-20 is the highest score on the board, so naturally it's targeted most often. Even with self-healing bristles, a board will wear out eventually (it tends to swell as fibers become dislodged). The treble-13, by contrast, is one of the least-used areas. When the board starts to show wear, you can rotate the metal ring with the numbers on it, moving the 20 to where the 13 was and giving your dartboard a new lease on life. This process can be repeated – commonly with the 2 and 7 – so a wire-numbered board might last four times as long as a painted version.