Let’s start with a pair of endings that many people find confusing: -able and -ible... They sound very similar when you say them and they share a main meaning, which is ‘able to be’:If you don't know the etymology, the following tips may help:
readable able to be read; easy to read eatable able to be eaten; fit to be consumed as food audible able to be heard collapsible able to be folded into a small space
Why are there two different endings that mean the same? It’s because of the route by which these endings found their way into English. The suffix -able comes from French -able or Latin -abilis, while the ending -ible comes from French -ible or Latin -ibilis.
...as a very general rule of thumb, if you choose -able, you’re more likely to be correct. This is because there are hundreds more words spelled with the suffix -able: our online dictionary of current English has around 180 adjectives ending in -ible, compared with over 1,000 that end in -able...Also note:
If the stem (the main part of the word that comes before -able or -ible) is a complete word in itself, then the ending is nearly always -able. A simple test is to take away the suffix – does the word still exist as an English word?
There’s a very small set of words which you can spell with either -able or -ible, such as extendable and extendible: both mean ‘able to be extended’ and both endings are acceptable. Any good dictionary will provide both spellings if they are equally correct.But...
Sometimes, the different spelling relates to a different meaning...The example given is contractable vs. contractible. Those few readers to whom this information is important should read the details and exceptions noted at the Oxford Dictionaries source.