The numbers in a dartboard are (sort of) arranged so that there is a
penalty for error. For instance, the number 20 at the top of the board is
sandwiched between the far smaller numbers 1 and 5. Aim at the 20 with
three darts and, if you are not very good, you risk a score of 20+ 5+ 1 = 26. Likewise, 19 is
anked by 3 and 7 (for a possible score of 29), and the 17 has
a 2 and 3 on either side (= 22).