All caps

In typography, ALL CAPS (short for "all capitals" or "all capitalized") refers to text or a font in which all letters are capital letters. "All caps" may be used for emphasis (for a word or phrase). They are commonly seen in the titles on book covers, in advertisements and in newspaper headlines. Short strings of words in capital letters appear bolder and "louder" than mixed case, and this is sometimes referred to as "shouting". All caps can also be used to indicate that a given word is an acronym.

Studies have been conducted on the readability and legibility of all caps text. Scientific testing from the 20th-century onwards has generally indicated that all caps text is less legible and readable than lower-case text. In addition, switching to all caps may make text appear hectoring and obnoxious for cultural reasons, since all-capitals is often used in transcribed speech to indicate that the speaker is shouting. All-caps text is common in comic books, as well as on older teleprinter and radio transmission systems which often do not indicate letter case at all.

In professional documents, a commonly preferred alternative to all caps text is the use of small caps to emphasise key names or acronyms, or the use of italics or (more rarely) bold. In addition, if all caps must be used it is customary to slightly widen the spacing between the letters, by around 10% of the point height. This practice is known as tracking or letterspacing. Some digital fonts contain alternative spacing metrics for this purpose.

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. Text in all caps is not widely used in body copy. The major exception to this is the so-called fine print in legal documents.

Capital letters have been widely used in printed headlines from the early days of newspapers until the 1950s. In the 1990s, more than three-quarters of newspapers in the western world used lower-case letters in headline text. Discussion regarding the use of all caps for headlines centers on the greater emphasis offered by all caps versus the greater legibility offered by lower-case letters. Colin Wheildon conducted a scientific study with 224 readers who analyzed various headline styles and concluded that "Headlines set in capital letters are significantly less legible than those set in lower case."

...
Wikipedia