A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007
Special cases
Spelling
Part III. Style 20 spelling
COMPOUND WORDS. For compound words consisting of two nouns, add s or es to the last noun.
bookkeepers
district attorneys
actor-singers
When a prepositional phrase or adjective follows, add s or es to the main noun.
sisters-in-law
attorneys general
men-of-war
LETTERS AND NUMERALS. In most cases, form the plurals of capital letters and numerals by adding s alone (not 's).
the three Rs
the 1950s
767s
With lowercase letters and some single capital letters, however, an s without an apostrophe can seem to create a different word (is, As) or an abbreviation (ms). If the s might create confusion, add an apostrophe. The apostrophe and s are roman even if the letter is italic (see 22.2.2).
dotting all the i's
x's and y's
three A's and two B's
ABBREVIATIONS. Form plurals of abbreviations without internal periods by adding s alone. If the singular form of the abbreviation ends in a period, put the s before the period. (See 24.1.3 on the punctuation of abbreviations.)
URLs
vols.
DVDs
eds.
Add an apostrophe before the s only when an abbreviation contains internal periods or both capital and lowercase letters.
PhD's (or Ph.D.'s; see 24.2.3)
A few abbreviations have irregular plurals (see also 24.7).
pp. (plural of p., page)
nn. (plural of n., note)
If you are writing in the sciences and using abbreviations for units of measure (see 24.5), use the same abbreviation for both the singular and the plural.
6 kg
37 m2
TERMS IN ITALICS AND QUOTATION MARKS. Form the plural of a term in italics by adding s alone (not 's) in roman type. Do not form the plural of a term in quotation marks by adding 's; rephrase the sentence.
two Chicago Tribunes
. . . included “To be continued” many times
not
. . . included many “To be continued's”