Horror aequi,[a] 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:
But following a to-infinitive, speakers will often use and instead of to:[8]: 236–42
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:
In some cases, the horror aequi principle is said to be responsible for stronger prescriptive rules, such as the ungrammatical use of verb + -ing followed by verb + -ing (gerund) where a to-infinitive is possible.[6]: 39
These pairs are grammatical:
However, when the first verb is in an -ing form, it is only grammatical to follow it with a to-infinitive. Thus, the latter of each pair is unacceptable:[6]: 39
*
It was beginning raining.*
They are continuing driving.(As is standard in linguistics, the asterisks indicate ungrammatical forms, while the question marks used in the same way below indicate forms that are grammatical but not considered acceptable.)
Horror aequi does not influence verbs that may only be followed by an -ing verb.
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.