These two suffixes are derived from Latin endings -abilis and -ibilis, either directly or through Old French. Of the two, -able is an active suffix that can be freely added to the stems of transitive verbs, whereas the set of -ible words is a closed one with meanings that are less susceptible to analysis. A few words exist in both forms (e.g. collectable and collectible); others appear to but differ in meaning (e.g. passable, passible). Most words are formed from verbs, but some are formed from nouns (e.g. comfortable, peaceable).
The primary meaning these suffixes convey is ‘able to be -ed’ or ‘capable of being -ed’, e.g. bearable, curable, manageable. Some meanings, however, are active rather than passive, e.g. agreeable = willing to agree, changeable = apt to change, comfortable = able to give comfort, viable = able to live. Others, such as reliable, are formed somewhat in defiance of syntax (here, on is suppressed). Many words acquire special meanings, e.g. actionable, appreciable, biddable, creditable, disposable, forgettable, incredible, noticeable, passable, remarkable, reprehensible, tolerable, unspeakable.
3. spelling problems.
Some formations require alteration of the stem, since both suffixes begin with a vowel. The most important are:
a Words in -y (preceded by a consonant) change y to i: dutiable, rectifiable, undeniable. But note employable, enjoyable.
b With some exceptions, words in silent -e lose the e when -able is added: adorable, excusable, lovable, losable, removable, usable. But note the preferred forms giveable, hireable, likeable, nameable, rateable, saleable.
c Words in -ce and -ge normally retain the e: bridgeable, changeable, chargeable, noticeable, peaceable.
d Words of more than two syllables ending in -ate lose this ending when -able is added: alienable (not alienatable), appreciable (has special meaning), calculable, demonstrable, estimable (has special meaning), inestimable (has special meaning), penetrable, tolerable. However, words of two syllables would not be viable without the ending and therefore retain it: creatable, debatable, dictatable, locatable, translatable.
e A final consonant is normally doubled when it is doubled in ordinary inflection: biddable, forgettable, regrettable.
f Words of more than one syllable ending in -fer double the r when the stress is on the final syllable, but do not when the stress is earlier in the word: conferrable, deferrable, offerable, preferable, profferable, sufferable (but in transferable the stress is variable on the first two syllables). Infer makes inferable and inferrable, and refer makes referable and referrable.
4. words ending in -able
admittable (also admissible)
contractable (of a disease; see also contractible)
defendable (in literal meanings; see also defensible)
educable (= able to be educated; see also educible)
expandable (also expansible)
extendable (also extendible, extensible)
impassable (= unable to be crossed; see also impassible)
persuadable (also persuasible)
5. words ending in -ible
admissible (also admittable)
contractible (= able to be shrunk; see also contractable)
defensible (of an argument etc.; see also defendable)
educible (= able to be educed; see also educable)
extendible (also extendable, extensible)
impassible (= unfeeling; see also impassable)
persuasible (also persuadable)