Words ending -able and -ible

Two common endings are -able and -ible. But how can you tell which one to use?
The difficulty is that we pronounce the leading vowels very similarly: we make a sort of -ugh- sound and you can't hear whether it is -a- or-i- .There are no simple rules to follow.
But here are three tips that you may find useful:
  • If the base word is not altered in any way then add -able.
    This works most of the time:
    understand    becomes    understandable,
    predict    becomes    predictable,
    comfort    becomes    comfortable,    ..... and so on.
  • There are many more words ending -able than -ible.
    I counted 900 -able words and 200 -ible words.
    So, if you make a guess and choose -able
    you're probably right 4 out of 5 times!
  • The best advice I can give you is to learn the most common -ible
    words and use -able for the rest.
    Very many of the -ible words are so rare that you'll never use them.
    Have you ever written fungible for instance?
Here are the most common -ible words:
edible: fit for eating
credible: can be believed
audible: can be heard
legible: easy to read
negligible: of no importance
terrible: causing terror
horrible: awful
feasible: can be done
visible: can be seen
sensible: reasonable
responsible: trustworthy
possible: liable to happen
digestible: can be digested
flexible: easily bent
permissible: allowable
compatible: suitable
Many of them have negative forms by adding in-, il-, ir-, or im- . For example:
inedible, illegible, irresponsible, impossible.
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