etymology - '-ible' suffix vs. '-able' suffix

This question comes about because I usually always spell the word incorrectly and the spell checker underlines in red the word: compatible.

In my head, I always want to spell it compatable, and my logic is as follows.

First, look at a definition:

Definition of compatible
a. - Capable of existing in harmony; congruous; suitable; not repugnant; -- usually followed by with.

There's a few words in there that make sense to my brain. Capable and suitable both have the -able suffix, because they can do something.

The root of the suffix is pretty self-explanatory. Able, suggestive of the ability to do whatever it is that the suffix is appended to.

Examples of words that make sense to me are:

• persuadable: the ability, or able-ness to be persuaded


• placable: [...] to be placed



• unappeasable: the inability, or unable-ness to be appeased

Other words of which the spellings are confusing to me: accessible, compatible, fallible, immersible

Why the -ible, and not -able when the end result seems to be expressing the same able-ness? To me, they should be spelled accessable, compatable, fallable, immersable.

What's the root of -ible being used and not -able?

Answer

I've always assumed that -able/-ible suffix patterns stem from the verbal conjugation of the Latin root, where 1st conjugation usually gives way to -able and the other three to -ible. Of course this goes for the Romance languages, not just English.

Disclaimer: this isn't a rule learned formally, more of an observation-based suspicion that never let me down through countless trials the SAT, my 7th grade spelling bee and that one semester when none of my courses were in English. Oh, Latin, where would I be without you? ;-)