The Indispensable Grammarian

A faithful reader provides the secret:


Rule
-ible -able
If the root is not a complete word, add -ible.

aud + ible = audible
Examples:

visible
horrible
terrible
possible
edible
eligible
incredible
permissible


If the root is a complete word, add -able.

accept + able = acceptable
Examples:

fashionable
laughable
suitable
dependable
comfortable
If the root is a complete word ending in -e, drop the final -e and add -able.

excuse - e+ able = excusable
Examples:

advisable
desirable
valuable
debatable


And, this being English, we must note that there are exceptions:

contemptible
digestible
flexible
responsible
irritable
inevitable


The joy this gave me, to know the rule at last: proof, as if more were needed, that I'm peculiar and needed long ago to get a real life.