American and British English Spelling Differences - Acronyms and Abbreviations

Acronyms and Abbreviations

Proper names formed as proper acronyms are often written in title case by Commonwealth writers, but usually as upper case by Americans: for example, Nasa / NASA or Unicef / UNICEF. This does not apply to most pure initialisms, such as US, IBM, or PRC (the People's Republic of China). However, it is sometimes done in the UK, such as Pc (Police Constable).

Contractions, where the final letter is present, are often written in British English without full stops/periods (Mr, Mrs, Dr, St, Ave). Abbreviations where the final letter is not present generally do take full stops/periods (such as vol., etc., i.e., ed.); British English shares this convention with the French: Mlle, Mme, Dr, Ste, but M. for Monsieur. In American and Canadian English, abbreviations like St., Ave., Mr., Mrs., Ms., Dr., and Jr., always require periods. Some initials are usually upper case in the US but lower case in the UK: liter/litre and its compounds ("2 L or 25 mL" vs "2 l or 25 ml"); and ante meridiem and post meridiem (10 P.M. or 10 PM vs 10 p.m. or 10 pm). Both AM/PM and a.m./p.m. are acceptable in American English, though AM/PM is more common.