All caps
In typography, all caps (short for all capitals or all capitalized; often written as ALL CAPS) refers to text or a font in which all letters are capital letters.
All caps is usually used for emphasis. It is commonly seen in the titles on book covers, advertising, billboards, and in dramatic newspaper headlines. Short strings of words in all caps appear bolder and "louder" than mixed case. All caps is also used to indicate that a given word is an acronym.
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Usage
Print media
Before the development of lower case letters in the 8th century, texts in the Latin alphabet were written in a single case, which is now considered to be capital letters. However, the shapes of words set in lowercase provide a valuable cue to readers that helps speed the process of reading; type in all caps forms a rectangular shape for every word, which makes distinguishing words harder. As a result, text in all caps is not widely used in body copy, as it is difficult to read in extended passages. The major exception to this is so-called small print in advertisements and legal documents, where the typesetter has incentive to make the text inaccessible.
Internet
This form of typography also appears in on-line forums. It was once an inevitable byproduct of using machines with limited support for lowercase text (such as certain dumb terminals, early Apple II models), but as full support of ASCII became standard, it became solely identified with "shouting" or attention-seeking behaviour. As a result, netiquette generally discourages the use of all caps.
Other
Writing in all caps (possibly with underscores replacing spaces) is an identifier naming convention in many programming languages that symbolizes that the given identifier represents a constant.