National Geographic Style Manual has moved to
1.
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
2.
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.