Letter numbers are the numbers that correspond the the placement of each letter in the alphabet. For example A has a letter number of 1 and Z has a letter number of 26. A (1), B (2), C (3), D (4), E (5), F (6), G (7), H (8), I (9), J (10), K (11), L (12), M (13), N (14), O (15), P (16), Q (17), R (18), S (19), T (20), U (21), V (22), W (23), X (24), Y (25), and Z (26)