Horror aequi

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.

Horror aequi in English

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. 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:

• 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, another strategy is to delay the second infinitive verb with intervening words or use an alternative infinitive clause. For example:

• I wanted to wait before starting the process.

Another reason why diligent writers may avoid the second to + verb structure is that it can be ambiguous. Since it is commonly interpreted as an elision of the first prepositional phrase from "in order to" it will avoid the oxymoronic interpretation "to wait in order to start the process" given that waiting and starting the process are contradictory.

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.

• ?The boy whom the girl whom 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 semantic difference can add some clarity:

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

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