The Editor's Manual
Grammar, usage, punctuation, and style resource for editors, writers, and learners of the English language.
BCE and CE are religiously neutral alternatives to BC and AD. While BC, BCE, and CE appear after the date (223 BC), AD appears before (AD 1776). Periods are optional and generally omitted.
Don’t enclose the abbreviation for a time zone in commas, and don’t use periods after the letters of the abbreviation. Full forms may or may not be capitalized. With an international audience, UTC offsets are often preferred.
Periods are used when the abbreviations for time are lowercased (a.m., p.m.) but omitted when they are capitalized (AM, PM).
The abbreviations “a.m.” and “p.m.” are generally lowercased in running text, though they may also be capitalized. When lowercased, the letters are followed by periods; when capitalized, periods are omitted.
Use “a.m.” and “p.m.” to indicate exact time, and use numerals instead of words to denote the hour and the minute. These abbreviations are generally lowercased in running text, but they may also be capitalized.
Each letter of an initialism is individually pronounced (e.g., USA). An acronym is pronounced as a single word (e.g., NASA). Contractions such as “Dr.” and “Mrs.” are also abbreviations, as are shortened forms like “vol.” and “Prof.”
The plural of an abbreviation is formed by adding an “s” (URLs, DVDs). Avoid inserting an apostrophe to form the plural. Abbreviated SI units do not have a separate plural form (1 cm and 10 cm). Abbreviated social titles such as “Mr.” and “Mrs.” have nonstandard plurals.
Periods are generally not inserted in abbreviations with more than one capital letter (MD, PhD). Contractions, shortened forms, and other abbreviations with lowercase letters usually contain a period (etc., p.m., Dr.). Don’t place a period after an abbreviated SI unit (cm, dl).
Provide the full form of an abbreviation at first use. Abbreviations that are acronyms or initialisms generally use all capital letters, while contractions and abbreviated Latin terms often contain lowercase letters. Use of “a” or “an” before an abbreviation depends upon how the abbreviation is pronounced.