Horror aequi

Horror aequi, or avoidance of identity,^: 100 is a linguistic principle that language users have psychological^: 266 or physiological^: 51 motives or limits on cognitive planning^: 51 to avoid repetition of identical linguistic structures.

The term originated in 1909 in Karl Brugmann,^: 219 who used it to explain dissimilation,^: 266 the tendency for similar consonants or vowels in a word to become less similar,^: 146 which can often be chalked up to simply "euphony".^: 219 Today, however, the term is usually applied instead to grammatical elements or structures.^: 219

One of the most widely cited definitions^{: 39 : 71} 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."^: 205

In the study of phonology, such avoidance falls under the obligatory contour principle,^: 100 which holds that certain consecutive identical sounds are not permitted^: 383–84 (such as in Mandarin Chinese, where two third tones are not used consecutively^: 104).

The term horror aequi is sometimes extended to the stylistic preference to avoid repeating the same word in a given text.