Dictionary.com Word FAQs
I always confuse the endings -able and -ible. Are there any useful tips to remember?
The adjective suffixes -able and -ible mean 'capable of', 'fit for,' or 'worthy of' and also 'tending to,' 'given to,' or 'causing'. The suffixes are pronounced the same, so they can be a spelling nightmare. There are no simple rules, but here are some useful tips. If the base word is not changed in any way then add -able. This works most of the time: comfort>comfortable, predict>predictable, understand>understandable. There are many more words ending -able than -ible. Words that can end in -ible often have negative forms that are created by adding il-, im-, in-, or ir- , e.g., illegible, impossible, inedible, irresponsible. Also, the suffix -able is often applicable for words that have noun forms ending in -ation, while the suffix -ible is often applicable to words that have noun forms ending in -ition, -tion, -sion, and -ion. One other helpful thing to remember is that -ible is not used after vowels, e.g., agreeable, permeable. The best advice is to learn the most common -ible words and use -able for the rest. And if you are unsure, look it up in a dictionary.