words in the same phrase, so one almost never encounters "U. S." When an abbreviation appears at the end of a sentence, only one period is used: The capital of the United States is Washington, D.C. Plural forms There is a question about how to pluralize abbreviations, particularly acronyms. Some writers tend to pluralize abbreviations by adding (apostrophe s), as in "two PC's have broken screens", although this notation typically indicates possessive case. However, this style is not preferred by many style guides. For instance, Kate Turabian, writing about style in academic writings, allows for an apostrophe to form plural acronyms "only when an abbreviation contains internal periods or both capital and lowercase letters". Turabian would therefore prefer "DVDs" and "URLs" and "Ph.D.'s", while the Modern Language Association explicitly says, "do not use an apostrophe to form the plural of an abbreviation". Also, the American Psychological Association specifically says, "without an apostrophe". However, the 1999 style guide for The New York Times states that the addition of an apostrophe is necessary when pluralizing all abbreviations, preferring "PC's, TV's and VCR's". Following those who would generally omit the apostrophe, to form the plural of run batted in, simply add an s to the end of RBI. RBIs For all other rules, see below: To form the plural of an abbreviation, a number, or a capital letter used as a noun, simply add a lowercase s to the end. Apostrophes following decades and single letters are also common. A group of MPs The roaring 20s Mind your Ps and Qs To indicate the plural of the abbreviation or symbol of a unit of measure, the same form is used as in the singular. 1 lb or 20 lb 1 ft or 16 ft 1 min or 45 min When an abbreviation contains more than one full point, Hart's Rules recommends putting the s after the final one. Ph.D.s M.Phil.s the d.t.s However, subject to any house style or consistency requirement, the same plurals may be rendered less formally as: PhDs MPhils the DTs. (This is the recommended form in the New Oxford Dictionary for Writers and Editors.) According to Hart's Rules, an apostrophe may be used in rare cases where clarity calls for it, for example when letters or symbols are referred to as objects. The x's of the equation Dot the i's and cross the t's However, the apostrophe can be