Plurals of Letters and Abbreviations
The plural of individual letters is normally written with -'s: there are two h's in this sentence; mind your p's and q's; dot the i's and cross the t's.
Some people extend this use of the apostrophe to other cases, such as plurals of numbers written in figures (e.g. "1990's"), words used as terms (e.g. "his writing uses a lot of but's"). However others prefer to avoid this method (which can lead to confusion with the possessive -'s), and write 1990s, buts; this is the style recommended by the Chicago Manual of Style.
Likewise acronyms and initialisms are normally pluralized simply by adding (lowercase) -s, as in MPs, although the apostrophe is sometimes seen. Use of the apostrophe is more common in those cases where the letters are followed by periods (B.A.'s), or where the last letter is S (as in PS's and CAS's, although PSs and CASs are also acceptable; the ending -es is also sometimes seen).
English (like Latin and many other European languages) can form a plural of certain one-letter abbreviations by doubling the letter: p. ("page"), pp. ("pages"). Other examples include ll. ("lines"), ff. ("following lines/pages"), hh. ("hands", as a measure), PP. ("Popes"), ss. (or ยงยง) ("sections"), vv. ("volumes"). Some multi-letter abbreviations can be treated the same way, by doubling the final letter: MS ("manuscript"), MSS ("manuscripts"); op. ("opus"), opp. ("opera" as plural of opus).
However often the abbreviation used for the singular is used also as the abbreviation for the plural; this is normal for most units of measurement and currency, as in 10 m ("10 metres").
Read more about this topic: English Plural
Famous quotes containing the word letters:
“... all my letters are read. I like that. I usually put something in there that I would like the staff to see. If some of the staff are lazy and choose not to read the mail, I usually write on the envelope “Legal Mail.” This way it will surely be read. It’s important that we educate everybody as we go along.”
—Jean Gump, U.S. pacifist. As quoted in The Great Divide, book 2, section 10, by Studs Terkel (1988)