Spelling: -ible vs. -able

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Rule

-ible

-able

If the root is not a complete word, add -ible.

aud + ible = audible

Examples:

  • visible
  • horrible
  • terrible
  • possible
  • edible
  • eligible
  • incredible
  • permissible

If the root is a complete word, add -able.

accept + able = acceptable

Examples:

  • fashionable
  • laughable
  • suitable
  • dependable
  • comfortable

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

excuse - e+ able = excusable

Examples:

  • advisable
  • desirable
  • valuable
  • debatable

Some exceptions:

  • contemptible
  • digestible
  • flexible
  • responsible
  • irritable
  • inevitable

You can try some exercises for spelling -ible and -able correctly at http://owl.english.purdue.edu/handouts/grammar/g_spelibleEX1.html.

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