Consider the following words:


The meaning of the prefixes is the same (negation the adverb), but they are still different prefixes.

How do I know which one to use? In other words, for an arbitrary adjective, how do I choose between "in-", "im-", "il-", and "ir-"?

The relevant discussion at ELU does not provide with any guideline.

  • Irregardless of these rules pay attention not to double the negative by using both a negative prefix and negative suffix. – SF. Jan 29 '13 at 8:41
  • @SF. correct, and also some stems are confusing: "important", "industrial" – bytebuster Jan 29 '13 at 14:39
  • Also "en-" and "em-", especially but not exclusively in words derived from Latin via French, e.g. "inquire" versus "enquire". – snailplane Feb 24 '13 at 17:14
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    The meaning of the prefix is negation in your examples; but it can also mean in, as in innate, impersonate, illuminate and irradiate. – StoneyB on hiatus Feb 24 '13 at 20:49

Formally, it is the same prefix "in-".
However, there are four different spellings that help pronunciation.

The rule only depends on the first letter of the adjective:

L — use "il-": illegal, illiterate;
P, M, or B — use "im-": improper, immediate, imbalance;
R — use "ir-": irresponsible, irrational;
other consonants or vowels — use "in-": indifferent, innumerable, inadequate;

  • +1 It's possibly worth adding that these uses are all adopted from or extensions of the practice in Latin, from which the prefix is borrowed, along with many of the words. – StoneyB on hiatus Jan 29 '13 at 13:51
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    I've never heard (or seen) "innatural"; "unnatural" is what I would expect. Webster's 3rd Unabridged shows fewer than four columns of "in-n*" words, and fewer than five of those words are negative, "innumerable" being the most familiar. For most, the "in" supplies the sense of "inherent". (However, if an "in" prefix is appropriate -- for negation or innateness -- the "n" should definitely be doubled.) – barbara beeton Jan 29 '13 at 13:54

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