What determines which prefix is attached to a word to form that word’s antonym? Why unlawful, but illegal? Why infirm, but impaired? You may be surprised that there’s a method to this madness: Negative prefixes come in forms that vary not only according to language derivation but also depending on meaning, and variations occur according to the letter that follows.
Here are the ins and outs of in- and un- and their like, and details about their distinctions:
These Greek-derived prefixes mark words expressing an absence of something (atypical, anodyne). A- is attached to words starting with consonants, and an- is the form for words beginning with vowels. An exception is words beginning with h; depending on the root word, either prefix may be present (ahistorical, anhydrous).
Anti-, from Greek by way of Latin, means “opposite” (antithetical) or “in opposition to” (antivirus), and can also denote defense (antisubmarine) or prevention (antidepressant).
This Latin prefix, when attached to a word, implies one of several meanings: absence (disaffected) or lack (disabled), opposition (disapprove), or removal (disenfranchise).
In- and its several variations, all signaling Latin derivation, also denote lack of a given quality. The variations il- and -ir are attached to words starting with l or r (illogical, irreversible). Im-, meanwhile, precedes m (immaterial) and p (impatient) and the lone example for b: imbalance. The rare prefix ig- sometimes comes before n (ignoble). In- appears before root words beginning with all other letters.
This Latin negative prefix is the least particular of the class, and can often be found attached to root words so that the resulting term differs in meaning from one formed by the attachment of another negative prefix to the same root word. For example, nonrational means “not according to rational means or rules,” but unrational refers to behavior that does not conform to these norms. The nonparticular non- is the go-to negative prefix for neologisms.
By contrast with the other negative prefixes, the attachment of this Latin-based form to a root word can, in addition to expressing lack or absence (unconcern), denote a reversal (uncoiled). It can also refer to an action not yet taken (unopened).
Other Usage Notes
The presence of negative prefixes can lead to awkward constructions such as “nonhearing-impaired people.” The easy solution in such cases is to relax the phrase: “people who are not hearing impaired.”
Note, too, that with most prefixes, the insertion of a hyphen is the exception, not the rule. Prefixed words should be closed except when the root word is a proper noun (non-Euclidean) or in the rare case when confusion with a similarly constructed but distinct word is possible (un-ionized and unionized, for example).
Wags have great fun with humorous poems and other compositions featuring invented examples of antonyms for unpaired words (words with negative prefixes that do not have antonyms), such as ept to contrast with inept — which actually has an etymological counterpart in apt — or ruth in opposition to ruthless.