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An abbreviation (from Latin brevis "short") is a shortened form of a word or phrase. Usually, but not always, it consists of a letter or group of letters taken from the word or phrase. For example, the word "abbreviation" can itself be represented by the abbreviation "abbr." or "abbrev."
An increase in literacy has, historically, sometimes spawned a trend toward abbreviation. The standardization of English in the 15th through 17th centuries included such a growth in the use of abbreviation. At first, abbreviations were sometimes represented with various suspension signs, not only periods. For example, specific phoneme sets like "er" were dropped from words and replaced with ɔ, like "mastɔ" instead of "master" or exacɔbate instead of "exacerbate". While this seems trivial, it was symptomatic of an attempt by people manually reproducing academic texts to reduce their copy time. An example from the Oxford university Register, 1503:
Mastɔ subwardenɔ y ɔmēde me to you. And wherɔ y wrot to you the last wyke that y trouyde itt good to differrɔ thelectionɔ ovɔ to quīdenaɔ tinitatis y have be thougħt me synɔ that itt woll be thenɔ a bowte mydsomɔ.In the 1830 in the United States, starting with Boston, abbreviation became a fad. For example, during the growth of philological linguistic theory in academic Britain, abbreviating became very trendy. The use of abbreviation for the names of "Father of modern etymology" J. R. R. Tolkien and his friend C. S. Lewis, and other members of Oxford literary group known as the Inklings, are sometimes cited as symptomatic of this. Likewise, a century earlier in Boston, a fad of abbreviation started that swept the United States, with the globally popular term OK generally credited as a remnant of its influence.
After World War II, the British greatly reduced their use of the full stop and other punctuations after abbreviations in at least semi-formal writing, while the Americans more readily kept its use until more recently, and still maintain it more than Britons. The classic example, considered by their American counterparts quite curious, was the maintenance of the internal comma in a British organization of secret agents called the "Special Operations, Executive" — "S.O.,E" — which is not found in histories written after about 1960.
But before that, many Britons were more scrupulous at maintaining the French form. In French, the period only follows an abbreviation if the last letter in the abbreviation is not the last letter of its antecedent: "M." is the abbreviation for "monsieur" while "Mme" is that for "madame". Like many other cross-channel linguistic acquisitions, many Britons readily took this up and followed this rule themselves, while the Americans took a simpler rule and applied it rigorously.
Over the years, however, the lack of convention in some style guides has made it difficult to determine which two-word abbreviations should be abbreviated with periods and which should not. The U.S. media tend to abbreviate two-word abbreviations like United States (U.S.), but not personal computer (PC) or television (TV). Many British publications have gradually done away with the use of periods in abbreviations completely.
Minimization of punctuation in typewritten matter became economically desirable in the 1960s and 1970s for the many users of carbon-film ribbons, since a period or comma consumed the same length of non-reusable expensive ribbon as did a capital letter.
A syllabic abbreviation (SA) is an abbreviation formed from (usually) initial syllables of several words, such as Interpol for International police, but should be distinguished from portmanteau words. They are usually written in lower case, sometimes starting with a capital letter, and are always pronounced as words rather than letter by letter.
They prevailed in Germany under the Nazis and in the Soviet Union for naming the plethora of new bureaucratic organizations. For example, Gestapo stands for Geheime Staats-Polizei, or "secret state police". This has given syllabic abbreviations a negative connotation, even though they were used in Germany before the Nazis, such as Schupo for Schutzpolizist. Even now Germans call part of their police Kripo for Kriminalpolizei. Syllabic abbreviations were also typical of German language used in the German Democratic Republic, for example, Stasi for Staatssicherheit ("state security", the secret police and secret service) or Vopo for Volkspolizist ("people's policeman").
Some syllabic abbreviations from Russian that are familiar to English speakers include samizdat and kolkhoz. The English names for the Soviet "Comintern" (Communist International) and "Milrevcom" (Military Revolution Committee) are further examples.
East Asian languages whose writing uses Chinese-originated ideograms instead of an alphabet form abbreviations similarly by using key characters from a term or phrase. For example, in Japanese the term for the United Nations, kokusai rengō (国際連合) is often abbreviated to kokuren (国連). Such abbreviations are called 略語 (略語) in Japanese. SAs are frequently used for names of universities: for instance, Beida (北大, Běidà) for Peking University (Beijing), Yondae (연대) for the Yonsei University, Seouldae (서울대) for the Seoul National University and Tōdai (東大) for the University of Tokyo.
There is never a period (full stop) between letters of the same word. For example, Tiberius is abbreviated as Tb. and not as T.b..
In American English, the period is usually added if the abbreviation might otherwise be interpreted as a word, but some American writers do not use a period here. Sometimes, periods are used for certain initialisms but not others; a notable instance in American English is to write United States, European Union, and United Nations as U.S., EU, and UN respectively.
A third standard removes the full stops from all abbreviations (both "Saint" and "Street" become "St"). The U.S. Manual on Uniform Traffic Control Devices advises that periods should not be used with abbreviations on road signs, except for cardinal directions as part of a destination name. (For example, "Northwest Blvd", "W. Jefferson", and "PED XING" all follow this recommendation.)
Acronyms that were originally capitalized (with or without periods) but have since entered the vocabulary as generic words are no longer abbreviated with capital letters nor with any periods. Examples are sonar, radar, lidar, laser, and scuba.
Spaces are generally not used between single letter abbreviations of words in the same phrase, so one almost never encounters "U. S.".
When an abbreviation appears at the end of a sentence, use only one period: The capital of the United States is Washington, D.C.
To form the plural of an abbreviation with periods, a lowercase letter used as a noun, and abbreviations or capital letters that would be ambiguous or confusing if the 's' alone were added, use an apostrophe and an s.
In Latin, and continuing to the derivative forms in European languages as well as English, single-letter abbreviations had the plural being a doubling of the letter, e.g. for footnotes.
Publications based in the U.S. tend to follow the style guides of the Chicago Manual of Style and the Associated Press. The U.S. Government follows a style guide published by the U.S. Government Printing Office.
However, there is some inconsistency in abbreviation styles, as they are not rigorously defined by style guides. Some two-word abbreviations, like "United Nations", are abbreviated with uppercase letters and periods, and others, like "personal computer" (PC) and "compact disc" (CD), are not; rather, they are typically abbreviated without periods and in uppercase letters. A third variation is to use lowercase letters with periods; this is used by Time Magazine in abbreviating "public relations" (p.r.). Moreover, even three-word abbreviations (most U.S. publications use uppercase abbreviations without periods) are sometimes not consistently abbreviated, even within the same article.
The New York Times is unique in having a consistent style by always abbreviating with periods: P.C., I.B.M., P.R. This is in contrast with the trend of British publications to completely make do without periods for convenience.
A period "within" a compound unit denotes multiplication of the base units on each side of it. Ideally, this period should be raised to the centre of the line, but often it is not. For instance, '5 ms' means 5 millisecond(s), whereas '5 m.s' means 5 metre·second(s). The "m.s" here is a compound unit formed from the product of two fundamental SI units — metre and second.
There should always be a (non-breaking) space between the number and the unit — '25 km' is correct, and '25km' is incorrect. In Section 5.3.3. of The International System of Units (SI), the International Bureau of Weights and Measures (BIPM) states "The numerical value always precedes the unit, and a space is always used to separate the unit from the number. … The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle."
The case of letters (uppercase or lowercase) has meaning in the SI system, and should never be changed in a misguided attempt to follow an abbreviation style. For example, "10 S" denotes 10 siemens (a unit of conductance), while "10 s" denotes 10 seconds. Any unit named after a person is denoted by a symbol with an upper case first letter (S, Pa, A, V, N, Wb, W), but spelt out in full in lower case, (siemens, pascal, ampere, volt, newton, weber and watt). By contrast g, l, m, s, cd, ha represent gramme, litre, metre, second, candela and hectare respectively. The one slight exception to this rule is that the symbol for litre is allowed to be L to help avoid confusion with an upper case i or a one in some typefaces — compare l, I, and 1.
Likewise, the abbreviations of the prefixes denoting powers of ten are case-sensitive: m (milli) represents a thousandth, but M (mega) represents a million, so by inadvertent changes of case one may introduce (in this example) an error of a factor of 1 000 000 000. When a unit is written in full, the whole unit is written in lowercase, including the prefix: millivolt for mV, nanometre for nm, gigacandela for Gcd.
The above rules, if followed, ensure that the SI system is always unambiguous, so for instance mK denotes millikelvin, MK denotes megakelvin, K.m denotes kelvin.metre, and km denotes kilometre. Forms such as k.m and Km are ill-formed and technically meaningless in the SI system, although the intended meaning might be inferred from the context.