In typography, all caps (short for all capitals; often written as ALL CAPS) refers to text or a font in which all letters are capitalized.
All caps is usually used for emphasis. It commonly appears in titles on book covers, on advertising billboards, and in dramatic newspaper headlines. It is not widely used in body copy.
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 (e.g. certain dumb terminals, early Apple II models), but as full support of ASCII became standard, it became solely identified with "shouting" or attention-seeking behavior. Because capital letters are supposed to be used for shouting, posting a forum message entirely in caps is a sign that the poster is a newbie, and more experienced users will sometimes jocularly use all-caps and misspelled phrases to make fun of newbies.
ALLCAPS or CAPS_WITH_UNDERSCORES is an identifier naming convention is many programming languages that symbolizes that the given variable is a constant.
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