Practice varies with regard to periods, with current usage increasingly in favor of omitting them, especially when the initialism consists entirely of capital letters. 3 Initialisms are made up of the initial letters of words and are pronounced as separate letters: CIA (or C.I.A.), NYC, pm (or p.m.), U.S. Another kind of contraction is the type with an apostrophe marking the omission of letters: can't = cannot, didn't = did not, you've = you have.

Such forms are invariably followed by a period. Sometimes shortenings are altered to facilitate their pronunciation or spelling: bike = bicycle2 Contractions are abbreviated forms in which letters from the middle of the full form have been omitted, for example, Dr. Some become the standard forms, and the full forms are then regarded as formal or technical, for example, bus = omnibus, taxi = taxicab, deli = delicatessen, zoo = zoological garden. Such shortenings are often but not always informal.

In the cases when they form words in their own right, the period is omitted, for example, hippo = hippopotamus, limo = limousine. 1 Shortenings of words usually consist of the first few letters of the full form and are usually spelled with a final period when they are still regarded as abbreviations, for example, cont. Types of abbreviations: There are four main kinds of abbreviations: shortenings, contractions, initialisms, and acronyms.

Good abbreviations for words