Word Endings — -ible vs -able

I was talking with my friend Liz the other day and she mentioned that someone had asked on the Glen Mitchell show about whether there was any rhyme or reason behind why some words end in “able” (such as “comfortable”) while others end in “ible” (such as “horrible”). Before I get to the answer, though, here’s the scoop on the Glen Mitchell Show for readers who aren’t in Dallas — the show airs on Dallas’ local NPR affiliate and each Friday show features call-in questions which other audience members are invited to help answer. It’s a bit like Ask.MetaFilter but on the radio (HHOS).

Anyhow, you wouldn’t think that there’d be any reason behind such word endings, but there actually is (well, mostly).

  • If the root is not a complete word, add -ible. (aud + ible = audible)

  • If the root is a complete word, add -able. (accept + able = acceptable)

For example, “laugh” makes sense on its own, so that becomes “laughable”. On the other hand, “elig” can’t stand on its own, so that one’s “eligible”. Then again, English just isn’t allowed to be that easy ;), so there are some exceptions, such as “irritable” and “inevitable”.

21 thoughts on “Word Endings — -ible vs -able

  1. :) This is one of the reasons I love the English language.

    p.s. The Thesaurus is one of my favorite books. I’m such a geek.

  2. the rule states:
    If the root is a complete word ending in -e, drop the final -e and add -able.

    i.e.
    excuse – e+ able = excusable

    therefore, reponse is an execption due to the fact that it uses ‘ible’ instead of ‘able’

    i.e.
    reponse – e + ible = reponsible

  3. The drop the silent-e rule applies whether adding ible or able:

    When adding a suffix that begins with a vowel to a root (Latin/Greek) or a root-word that ends with a silent-e, drop the e. Both ible and able begin with a vowel.

    spons and spond are two spellings of the same Latin root.

    accept is not a root-word:
    prefix ac + Latin root cept = accept

  4. There is a different generalization that helps for many, but not all, words like irritable.

    If you can make a word ending in ation, able is usually the correct spelling:
    irritation irritable
    vegetation vegetable

    If you can think of a word ending in ition or tion, ible is usually the correct spelling:
    inflection inflexible (flex/flect same root)
    audition audible
    admission admissible

  5. response is the root word. It isn’t an exception. There is another rule also states that you drop the final e before adding the suffix -ible or -able

    so you get response – e + able = responsable

  6. Posy, responsible is an exception to the superficial rule that states: “we should add +able to roots that are complete words. And here we are. Response is a complete word, our root in this case, and we drop the final silent -e as required. So why do we get -ible instead of -able? To figure this out let’s look deeper into the etymology of the word and notice that Latin words ending in -ERE, -IRE all get -IBLE suffixes in Modern english. At least in the examples I have researched myself. E.g. contemptIBLE (from contemnere), digestIBLE (from digerere), flexIBLE (from flectere), responsIBLE (from respondere) and watch out! irritABLE (irritARE), inevitABLE (evitARE). Does that answer your question?

  7. Perhaps the reasoning behind the particular “exceptions” has to do with the implications of the ible/able endings vs. the root word. For example, response has little to do with the word responsible.

  8. In response to #9, the “e” is retained after the “g” in manageable, to keep the “j” sound of the g. Try saying “managable” – man-a-gable instead of man-a-jable.

  9. None of this explains “deductible” nor “suggestible” which are the two words that led me to this discussion…

  10. Indispensable – still doesn’t look right to me. Probable, which sadly doesn’t mean ‘able to be probed’.

  11. Well, I came here because I was looking at adjectives used on furniture.

    Retractable
    Collapsible (Collapsable is an accepted variant)

    Looking at the two rules provided here and in the comments.
    Retract is a full word – add able. Checks out.
    Retraction hints at add ible. Does not check out.

    Collapse is a full word – add able. Does not check out.
    Cannot form a -ation or -ition version.

    The dictionary doesn’t provide much guidance either, except to say able implies ability or susceptibility (why not susceptability?).

    I think the take away lesson from the dictionary is that -ible is a variant of -able, and thus no good rule is going to cover it all.

    The best rule is to be understanding of any spelling errors that result from this infernal suffix.

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