What determines whether a verb-to-adjective transformation ends with the suffix -able (as in assessable) or with -ible (as in accessible)? Why do some root words accept both options? What happens if the word ends with an e? Answers to these and other questions about -able and -ible follow.
The suffixes -able and -ible both express capability, fitness, or worth (or mean “tending, given, or liable to”), but there’s one key practical difference: The former flourishes, and the latter has fossilized. New words can be formed by attaching -able to an adjective (I’ll get back to that in a moment) — or a noun (more about that later, too) — but -ible, though widespread in existing words, is discouraged for new coinages.
Some words use one form unequivocally. (For example, immovable and invincible are never rendered immovible and invincable). Others are spelled either way, although one form predominates (as in the case of discernible and its less frequent variant discernable). In some cases, the variants reflect a distinction of meaning: For example, collectable means “able to be collected,” but the more common collectible has the connotation of desirability and is used as a noun to denote something worth collecting.
Intransitive verbs can also be transformed into adjectives by appending -able. Strictly speaking, reliable, for example, means “able to be relied on,” not just “able to be relied,” but the needs of the language have silenced opposition to such usage. Adjectives are also formed from attaching -able to nouns, such as objectionable from objection, though the nonword objectable is the logical formation based on the verb-plus-able formula.
Two other peculiarities exist regarding the suffix: When it is appended to a verb ending in -ate, such as calculate, the original suffix is omitted, resulting, for example, in calculable (which is overshadowed in frequency of usage by its antonym, incalculable). And when a word ends in e, such as in the case of move, the e is omitted when -able is attached, hence movable. (You’ll see moveable and the like in older publications, but this form is rare in contemporary usage.) Exceptions occur when a soft c or a soft g precedes the e, as in serviceable and changeable.
When coining new terms, keep these rules in mind — though consider, as well, that even some existing words, such as embraceable, are ungainly, and newly minted terms may be disagreeable to some readers.