Why Do Words End In -Able Or -Ible?

Why Do Words End In -Able Or -Ible?
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I saw the sign pictured above over the Christmas break (hi Yarragon!), and realised something: I didn’t know if there was any underlying principle to when a word is spelled with -able or -ible on the end. It was time to find out.

The Macquarie Dictionary notes that while collectable is the dominant spelling, collectible is also acceptable. It also offers the following note on the variant spellings:

The suffix -able is the form used with words of non-Latin origin (as in knowable, readable but Latin words borrowed into English come with either -able or -ible depending on the stem of the root word in Latin. The two are interchangeable in some derivatives, especially those which have been formed in modern English.

Given that almost no-one speaks Latin these days, that’s not necessarily a good basis for distinguishing which one to use. The fact that exceptions are becoming increasingly common reminds us that etymology is an interesting hobby, but isn’t always a helpful guide to meaning or spelling.

The most useful rule you can deduce from this is that if you’re unsure which spelling to use, -able is more likely to be correct. But if you’re not sure, check in a dictionary (or pick an alternative word which you can spell). Accuracy matters.

Lifehacker’s Mind Your Language column offers advice on improving your writing.


  • The -able/-ible dichotomy has nothing whatsoever to do with accuracy. It is one of the many totally pointless English spelling hassles which i have summarised on my EnglishSpellingProblems blog and which serve no purpose other than to make learning to spell more difficult than need.

    In the 7,000 most used English words which i analysed, 26 end in -able, 17 in -ible. The -ible endings make sense after g (eligible, illegible/legible, tangible) to keep the g soft. The rest would all be better with -able (audable, compatable, edable…), on the model of ‘doable’, in the sense that u are able to do something.

  • ible words are from latin. able words usually of french origin. take away the able and you are usually left with the complete root word.
    some able words come from bad go getter concepts. doable is not a word. try it can be done.

    although it could be achievable (hmm) – but that sounds weasely (and not a good Ron type of Weasley)

  • So, let be get this straight . . . . there is no rule . . . . it is just that one spelling is more common than the other. “In England driving on the left is more common that driving on the right” – is that it?

    Learning Latin and Medieval French on top of Anglo-Saxon and Norse does not seem to be of any help.

    English in its written form seems to be cruel and “if you’re not sure, check in a dictionary” is clearly not a solution. Would someone just write out a simple rule – that has no exceptions – and then we can get on and occupy our selves with genuinely important matters. Surely that is doable?

  • I remember being amazed when learning German in high school that the grammatical and pronunciation rules are… rules. If it’s spelled “ie”, it’s pronounced “ee”, if it’s spelled “ei” it’s pronounced “eye”

    On the other hand, having had the good fortune to learn English by immersion as a native language, I love the huge range of options and pun and wordplay possibilities that such a sprawling, inexact tongue allows.

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