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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?

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    All I know is that both forms already existed in Latin: see mirabilis vs. horribilis. The distinction probably originates in some property of Latin phonology, with -able later also being applied to all(?) words that have no Latin origin (doable, approachable, etc.). – biziclop Sep 25 '15 at 21:00
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    Please see english.stackexchange.com/a/66463/58761 – anongoodnurse Sep 25 '15 at 23:05
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    @JohnLawler Not all of our -able words are Germanic. Some are from Latin as well, since they were -abilis words from verbs in the first conjugation, yielding things like separable, operable, provable, culpable, portable, eliminable, ignorable, potable, mutable. But even here there are historical anomalies like capable. Still, -able is productive and -ible is not, so -able words will always outnumber -ible words. Plus some of the older -ible words get respelled following the productive model, like inferrible becoming inferable. – tchrist Sep 27 '15 at 15:09
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    Food for thought: Both "passable" and "passible" are English words, but they mean very different things. (I suppose there must be other examples like this, but I don't know any offhand.) – Andreas Blass Sep 27 '15 at 20:20
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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? ;-)

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