Many pairs of words use un- as a prefix for the preferred adjective but in- as a prefix for the preferred noun (e.g. unstable/instability, unequal/inequality, unable/inability, unjust/injustice, ungrateful/ingratitude).

I was wondering if anyone has encountered the reverse phenomenon? Where the in- is a prefix for the preferred adjective but un- is the prefix for the preferred noun? If not, any explanations?

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    Note that all these nouns were borrowed from Latin, which used the in- negative. These adjectives, on the other hand, developed from the nouns in English and take the English un- negative. There are plenty of native English pairs like unhappy/unhappiness, as well as borrowed Latin pairs like ingrate/ingratitude. – John Lawler Mar 27 '13 at 21:07
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    I.e, it doesn't matter whether it's a noun or an adjective; what matters is whether it's native or borrowed. – John Lawler Mar 27 '13 at 21:08
  • invaluable is an in-adjective without a noun. – Jakob Weisblat Mar 29 '13 at 15:51
  • I believe one of the reasons one does not see many adjectives with in- prefixes for negation is that there is the confusing alternative of in- as an emphasizing (morphological) unit. For example, both inflammable and flammable describe something which is capable of burning, the former more so than the latter. – jbeldock Apr 8 '13 at 22:53
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    @JohnLawler I'd consider your two comments a satisfactory answer - that it's about the origin of the word rather than its role in a sentence, the difference between Latin and Germanic words. – njd Apr 18 '13 at 11:26

The only case I can come up with is ingrate/ungratefulness. According to wiktionary, ingrate can be used as an adjective, they quote no lesser source than Shakespeare:

But I will lift the down-trod Mortimer / As high in the air as this unthankful king, / As this ingrate and canker'd Bolingbroke. — William Shakespeare, King Henry IV, Part 1

So, even though ingrate is more often used as a noun, it can be an adjective and if coupled with ungratefulness breaks your rule.

  • Yes! Archaic but a perfect example of the phenomenon going the other way. Great find. – highlyverbal Jun 10 '13 at 0:08

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