Several rules govern the correct formation of plural nouns. For most nouns, simply add s: dogs, houses, meteors. Nouns ending in soft ch, j, ss, x, or z add es: churches, glasses, axes, buzzes. Words ending in o preceded by a vowel add s: cameos, tattoos. Words ending in o preceded by a consonant add es: heroes, potatoes. Pianos is an exception. When two plurals are listed in Webster’s Eleventh, use the first. Apply the following rules to unusual plural formations:
Unless there is likelihood of confusion, add only an s to form the plurals of numbers, acronyms, and single or multiple letters used as words.
the 1940s
YMCAs
’ 40s
ABCs
6s and 7s
TVs
2x4s
the three Rs
dos
don’ts
Use an apostrophe and an s to form the plurals of lowercase letters used as nouns.
x’s and y’s
p’s and q’s
Use an apostrophe and an s to form the plurals of capital letters that would be confusing if s alone were added.
S’s and A’s
Use an apostrophe and an s to form the plurals of abbreviations with lowercase letters.
p.m.’s
Use an apostrophe and an s to form the plurals of abbreviations with internal periods.
M.A.’s
f.o.b.’s
Use an s to form the plurals of abbreviations with no internal periods.
IQs
Btus
OKs
YMCAs
Also see “plurals” in the Abbreviations section of the BHGStylebook.