Should That Be –Able or –Ible?

Or even –uble?

Perhaps not one of the burning questions of the day, but I bet you’ve hesitated over this at least once.

I did recently, when trying to Google an antiques dealer who had a well-priced early 20th-century silver snuffbox. The dealer used the form collectibles in its business name, it turned out.

This is the (mostly) US spelling of the preferable (but disappearing) collectable, which means ‘that may be collected’ (Violators are subject to a fine collectable on summary conviction) or ‘(thing) worth collecting, sought after by collectors’ (Silver snuffboxes are highly collectable – really!).

Is there a general rule for determining which ending to use? Most of the time it’s –able, and usually if the stem is a complete word in itself (bill/billable, as opposed to feasible or tangible, which aren’t formed from feas and tang).

The more detailed rules are a bit arcane.

The default position is –able, unless the word is derived from a Latin verb ending in –ere or –ire, or where there is a well-established ­ible form (like collectible?) in English already.

If your Latin is a bit rusty, you’ll just have to remember a list of words like this:

  • demonstrable (–able replaces the final –ate in demonstrate, as with abominate, alienate, appreciate, calculate etc.)
  • enforceable (the final –e of enforce stays so the following consonant stays soft; same thing for manageable, pronounceable etc.)
  • forgivable (the final –e in forgive drops out because it isn’t necessary for pronunciation; same thing for movable, usable and the like)
  • justifiable (the final –y gets converted to an i)
  • predictable


  • convertible
  • incorrigible (compare uncorrectable)
  • incredible (compare unbelievable)
  • irresistible
  • perceptible (perceivable)
  • responsible
  • reversible
  • soluble and insoluble (a soluble substance, an insoluble problem – although unsolvable is also possible for the latter)

Where the stem ends in a hard C or G, or a consonant that gets doubled in the adjectival form, go with –able (amicable, navigable, forgettable).

New words have tended not to go with the –ible ending.

Sometimes there are two forms, not always with the same meaning: extendable and extendible are interchangeable; but contractable (‘able to be caught’, like a disease) isn’t the same as contractible (‘capable of being made smaller’).

Totally confused now?

Next: your queries answered, part 3

Neil Guthrie (@guthrieneil)


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