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, 2013 at 8:41
  • @SF. correct, and also some stems are confusing: "important", "industrial" Jan 29, 2013 at 14:39
  • Also "en-" and "em-", especially but not exclusively in words derived from Latin via French, e.g. "inquire" versus "enquire".
    – user230
    Feb 24, 2013 at 17:14
  • 2
    The meaning of the prefix is negation in your examples; but it can also mean in, as in innate, impersonate, illuminate and irradiate. Feb 24, 2013 at 20:49

1 Answer 1


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
    +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. Jan 29, 2013 at 13:51
  • 5
    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.) Jan 29, 2013 at 13:54

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