Is there any general rule how to choose between negation prefixes?

Sometimes it's un- like in unpopular or unhealthy, while in other cases it's in- (and its variants) like in impossible or irrelevant.


2 Answers 2


This was a thought provoking question, but after some thinking I have to say I do not believe there is any rule that covers the majority of cases. It just needs to be learned through practice.

  • that's right. There are rules, but there are exceptions; some rules depend on knowing the etymology of a word which often you won't know. So your best bet, unfortunately, is to listen to native English speakers ( and read native writers) and note which words take which negation prefix. Remember you don't have to learn ALL of them; only the ones you use. And most native speakers will good-naturedly correct you if you slip up (or guess wrong) now and then. Aug 10, 2015 at 7:25

If the adjective is Latin or French you may assume that the prefix is in- or one of its variants (n easily changes, influence of the following consonant). Possibilis/e is a Latin adjective, so you have possible and impossible.

"healthy" is neither Latin nor French so you have "unhealthy".

Added: This is only a rule of thumb. There are cases where you have un- + a Latin adjective. See comment below. So when in doubt you have to consult the dictionary.


My assumption is when the negative adjective was alredy a current word in Latin or is in the Latin dictionary then we have the prefix in- and its variants im/il/ir-. If the word was formed later, maybe around 1600 as unpopular then the English prefix un- is used. But I admit I am not sure whether this assumption holds water.

  • 4
    Doesn't seem like a great rule to go by to me -- what about "popular?" Unpopular is the correct negation of that, but popular derives from Latin as well. In my opinion this is "one of those things" in English that you simply have to memorize for each given word, unfortunately (there's another one for you).
    – Crazy Eyes
    Jul 13, 2015 at 20:18

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