The choice of -able vs -ible is horrendous. It defeats even me. Compressible or compressable? Collectible or collectable? There are some rules, and there is some guidance for when there are no rules. In the above two cases you can use either. And I'm not going to make you learn any Latin.

We actually have three endings:

  1. -able from Latin -abilis (muto 'I change' --> mutabilis 'changeable, mutable')
  2. -ible from Latin -ibilis (credo 'I believe' --> credibilis 'believable, credible')
  3. English able used as a suffix: breakable, un-put-down-able
First we'll dispose of some easy cases.

Where the word has no pretensions at all of coming from Latin, use -able. That's the third case above.

Where the preceding consonant changes, that's pretty much a guarantee you've got a Latin word that takes -ible. This is because of a thing in Latin grammar that you don't need to know about. Really, you don't. The consonant change is enough of a sign. Here are some examples.

Now, so far the examples have been based on verbs: breakable is from break, permissible is from permit, and so on. Where the Latin verb has not been borrowed into English, it seems they all take -ible. I won't swear to its being all, and I can't strictly justify this in Latin grammar, but it holds for many common ones: audible, visible, horrible, terrible, edible, possible, and so on. There are no English verbs that these come from, no words *aud, *vis, *horr, *ed.

If you like, you can take the previous case as more examples of this: there are no English verbs *comprehens or *permiss either. And also fallible, where there isn't a word *fall with that vowel.

That leaves a large number of pairs like collapsable/collapsible, collectable/collectible, exhaustable/exhaustible, connectible/connectable, and so on and so on, where they both look right (or wrong) no matter how long you stare at them. There are two ways of approaching this. Three ways.

  1. Decide it isn't important and write whatever you like.
  2. Look them up again and try to memorize them. (This is what I do and I can't get it to work.)
  3. Bear in mind that any English verb is allowed to take the English suffix -able. So even if expressible is the better Latin, you are allowed to coin the word expressable.
There is one rule for the negative. The pure Latin ending -ible ought to take the Latin negative in-, not the native English un-. So inaudible, invisible, incomprehensible, indefensible, and so on. This can even help to decide on the ending: do you say unexhaustable or inexhaustible? The second, of course, so it's Latin with -ible.

The above are rules of thumb only. They work in the majority of cases, and will not lead you to too many howlers.

One example of the rules failing is gullible, which is from a native English word, not Latin, but has -ible.

-a*ble (-a*b'l). [F. -able, L. -abilis.]

An adjective suffix now usually in a passive sense; able to be; fit to be; expressing capacity or worthiness in a passive sense; as, movable, able to be moved; amendable, able to be amended; blamable, fit to be blamed; salable.

The form -ible is used in the same sense.

⇒ It is difficult to say when we are not to use -able instead of -ible. "Yet a rule may be laid down as to when we are to use it. To all verbs, then, from the Anglo-Saxon, to all based on the uncorrupted infinitival stems of Latin verbs of the first conjugation, and to all substantives, whencesoever sprung, we annex -able only." Fitzed. Hall.

 

© Webster 1913

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