letters of the alphabet

Letters referred to as letters are set in italic, as are letters that represent value in mathematical equations. Plurals add an italic apostrophe plus a roman s:

the letter p
plural with es
x squared
Greek letter chi
his Scottish r's
P = 2 l + 2 w
there are two c's and two m's in accommodate

Letters in idiomatic expressions, as nouns, and as musical notes are set in roman.

Plurals add an apostrophe plus s:

ABC's
B-flat major
B movies
f-word
key of C minor
p's and q's
Model T's
S-shaped curve
straight A's
three R's
U-turn
V-formation
X marks the spot

See also abbreviations and plurals.