D is the 4th letter of the alphabet. G the 7th and J the 10th. So which letter comes 13th in the ABC's? 1A 2B 3C 4D 5E 6F 7G 8H 9I 10J 11K 12L 13M. This also happens to be the middle of the letters. The numbers are increasing by 3 each time.
The next value is M.
