The suffixes -able and -ible both mean capable of or suitable for, but we treat them differently. The most important difference is that -able is a living suffix, meaning we can affix it to virtually any verb without using a hyphen, while -ible is not used to make new words. It lives on mainly in old words passed down through the centuries.
As the living suffix, -able is useful for coining new words, though we often have to ignore spell check when it comes to -able coinages. For example, our spell check disapproves of sanctionable, channelable, overthrowable, redoable, and torturable, but these are perfectly good words and do not require hyphenation.
To form an -able word, treat the verb as you do when making an -ing participle. For example, we make moving from move by dropping the e and adding -ing. So, to make move‘s -able adjective, we drop the e and add -able: movable.
But when creating an -able word, make sure there is not already an equivalent -ible word (see below). For instance, convertable is superfluous because we already have convertible. All accepted -ible words are listed in the dictionary.
Unlike -able, -ible isn’t used to make new words. It exists only in words retained from earlier stages of English. Here are a few of the most common words with the suffix:
Many -ible adjectives have corresponding -able words from which they have differentiated over time. For example, forcible and forceable have different meanings. Other -ible words have -able variants that are identical in meaning—for example, extendable and extendible.