Abbreviations often consist of the first letter of a word, or each important word in a group, written as a capital followed by a period. For example, P.O. stands for post office and R.A.M.C. for Royal Army Medical Corps. Sometimes an abbreviation is printed as a small letter and period, as in b. for born and m. for married. The same abbreviation may be used for different words; for example, m. may also stand for masculine or meter. The reader usually can determine the correct word from the context.

Other letters of a word may be added, as in ms. for manuscript and ft. for foot. Initial letters or syllables sometimes form a new word, as NATO (North Atlantic Treaty Organisation) or OPEC (Organisation of Petroleum Exporting Countries). Such abbreviations, called acronyms, are not followed by periods. Letters in abbreviations may be doubled for the plural form, as in ll. for lines and pp. for pages. For certain frequently used abbreviations such AM, PM, AD, BC, and decorations, small capital letters are usually used instead of large capitals.

Abbreviations are most often used for common words such as the names of days, months, and counties. Long words and phrases are often abbreviated such as Lieut. for Lieutenant and C.R.O. for County Record Office. Academic degrees and titles are usually abbreviated, as in D.D. for Doctor of Divinity and H.R.H. for His (or Her) Royal Highness. In business, Co. is used for Company, and Ltd. for Limited.

In most Latin phrases in common use, only the first letter of each word is used, as in n.b. for nota bene ("notice well") and i.e. for id est ("that is"). An exception is etc. for et cetera ("and others").

The use of full stops with abbreviations is today largely optional. Full stops are most commonly used with lower-case abbreviations (a.m., etc.), though not with truncated words like `exam' and `fridge'. For example, PA (public-address system) but p.a. (per annum). It is advisable to spell out all but the most familiar abbreviations on introducing them within your own documents.

Acronyms are words formed from the initial letters and/or syllables of other words, intended as a pronounceable abbreviation ; for example, NATO (North Atlantic Treaty Organization), radar (radio detecting and ranging), RAM (random access memory) and FORTRAN (formula translation).

Many acronyms are so successfully incorporated into everyday language that their origin as abbreviations is widely overlooked. Full stops are not normally used in acronyms.

Our guide contains abbreviations gathered over many years and from many sources. Some - such as railway company abbreviations - may not be universally known, but may yield a clue to the identity of a document, ticket, badge, rubber stamp mark, etc.