Horror aequi,^{[lower-alpha 1]} or avoidance of identity,^[2]: 100 is a linguistic principle that language users have psychological^[3]: 266 or physiological^[1]: 51 motives or limits on cognitive planning^[1]: 51 to avoid repetition of identical linguistic structures.

The term originated in 1909 in Karl Brugmann,^[4]: 219 who used it to explain dissimilation,^[3]: 266 the tendency for similar consonants or vowels in a word to become less similar,^[5]: 146 which can often be chalked up to simply "euphony".^[4]: 219 Today, however, the term is usually applied instead to grammatical elements or structures.^[4]: 219

One of the most widely cited definitions^{[6]: 39 [7]: 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."^[8]: 205

In the study of phonology, such avoidance falls under the obligatory contour principle,^[2]: 100 which holds that certain consecutive identical sounds are not permitted^{[9][10]: 383–84} (such as in Mandarin Chinese, where two third tones are not used consecutively^[11]: 104).

The term horror aequi is sometimes extended to the stylistic preference to avoid repeating the same word in a given text.^[12]

One illustration of horror aequi in English is the use of and + verb rather than the typical to + verb following certain to-infinitive verbs such as wait, try and check in order to avoid repeating the to + verb pattern.^{[8]: 236–42} Thus, speakers typically use:

• I'll try to find a solution.
• I tried to open the door.
• I'll wait to hear the answer.
• I waited to start the process till the audience was assembled.

But following a to-infinitive, speakers will often use and instead of to:^{[8]: 236–42}

• I'm going to try and find a solution.
• I wanted to try and open the door.
• I'm going to wait and hear the answer.
• I wanted to wait and start the process.

In addition to using and instead of to in order to avoid horror aequi, a strategy is to delay the second to + verb with intervening words.^[8]: 236 For example:

• I wanted to wait for a few minutes to start the process.

Other examples clearly demonstrate how horror aequi helps prevent confusion. Sentences with repetitive words or forms can be nearly incomprehensible even when adhering to grammatical rules.^[2]: 101

• ?The boy who the girl who the other boy had hit had called came running.

Confusion here comes from both the repeated embedded who relative clauses and from the lack of semantic variety. Merely adding sematic difference can add some clarity:^[2]: 101

• ?The number that the girl who the horse had kicked had called was for animal control.

The horror aequi principle holds that both of these examples would be avoided.

• Uses of English verb forms
• Elegant variation – Use of synonyms to avoid repetition