Horror aequi

Horror aequi, also known as identity avoidance or avoidance of identity, is a linguistic principle that language users have psychological or physiological motives or limits on cognitive planning to avoid repetition of identical linguistic structures.

The term originated in 1909 in Karl Brugmann, who used it to explain dissimilation, the tendency for similar consonants or vowels in a word to become less similar, which can often be chalked up to simply "euphony". Today, however, the term is usually applied instead to grammatical elements or structures.

One of the most widely cited definitions is that of Günter Rohdenburg: "the horror aequi principle involves the widespread (and presumably universal) tendency to avoid the use of formally (near-)identical and (near-)adjacent (non-coordinate) grammatical elements or structures."

In the study of phonology, such avoidance falls under the obligatory contour principle, which holds that certain consecutive identical sounds are not permitted (such as in Mandarin Chinese, where two third tones are not used consecutively).

The term horror aequi is sometimes extended to the stylistic preference to avoid repeating the same word in a given text, also known as elegant variation.