12.3a Letters joined to words or other letters by a hyphen. [II.12.a(4)]

A letter sign precedes any single letter that means a letter when it is joined to a word by a hyphen so that it won't be misread as a one-cell, whole-word contraction. When a hyphen or a dash joins letters of the alphabet, a letter sign precedes each letter. Examples:

The letters a-jSeats M--S saved