Availability or Availibility? How to Spell -ability and -ibility Words

It is availability, with an a: a-v-a-i-l-a-b-i-l-i-t-y. The word is built from available, and available ends in -able, so the noun ends in -ability. "Availibility" is a misspelling, and a very common one, which is probably why you are here.

The rule behind it fits in one sentence, and it holds better than almost any rule in English spelling: a noun ending in -bility keeps the vowel of the adjective it came from. Available gives availability. Responsible gives responsibility. Possible gives possibility, and probable gives probability. You never have to decide how to spell the noun. You only have to know the adjective.

Below: why availability in particular pulls people toward an i, how far the rule holds when you check it against whole dictionaries, and the one family of related words that can make the hidden vowel audible.

The rule in one line

Every one of these nouns is made the same way. Take the adjective, change its last two letters from -le to -il, and add -ity. The vowel in front of the b is never touched.

So available becomes availabil- and then availability. Responsible becomes responsibil- and then responsibility. Nothing else moves, and that is the whole trick: the a or the i you see in the noun is a copy of the a or the i already sitting in the adjective.

The pattern is wider than -able and -ible. Noble gives nobility, soluble gives solubility, and stable gives stability. Whatever vowel stands before the b in the adjective stands before the b in the noun.

Why availability tempts you toward an i

Say availability aloud and listen. Written out in everyday letters, Merriam-Webster's pronunciation is roughly uh-vay-luh-BIL-uh-tee. The stress falls on BIL, so the loudest vowel in the word is an i. The syllables on either side of it collapse into a soft "uh." That "uh" is a schwa, the neutral vowel English uses in almost every unstressed syllable, and it sounds the same whichever letter is written. The a you need is one of those soft syllables, sitting right next to the one loud i.

The likely result is that the ear supplies the vowel it heard most clearly. Availibility repeats the i of the stressed syllable, and the run of similar beats, luh-BIL-uh, makes the repetition feel natural. We explain the sound itself, and why it causes so many adult spelling errors, in schwa, the lazy vowel.

With many schwa words, the cure is a longer relative in which the vowel is stressed and audible: grammatical rescues grammar. That cure does not work here. In available, uh-VAY-luh-bul, the same a is a schwa too, and in availability the stress jumps to the i of BIL, which was never in doubt. Neither word ever says that a out loud. Your ear cannot settle this one, and you need the structure instead.

There is one letter-level hook worth keeping, because it shows you the a instead of asking you to hear it: availability is avail followed by the whole word ability, a-v-a-i-l plus a-b-i-l-i-t-y. Ability is a short everyday word most people spell without a second thought, so once you see it sitting inside the longer word, the a comes with it.

There is a second pull for anyone who also speaks Spanish or French. The Spanish word for availability is disponibilidad, and the French is disponibilité, both with an i before the b. For responsibility the pull runs the other way: Spanish responsabilidad, French responsabilité, Portuguese responsabilidade, all with an a. Spanish and French speakers can be pulled toward the wrong vowel on both of the words they are most likely to write at work.

How far the rule holds

A rule is only worth learning if it survives contact with real words, so we tested this one. The open-source American and British spell-check dictionaries used by LibreOffice contain more than 400 nouns ending in -ability or -ibility. For every noun whose adjective is in standard use, the vowel matched the adjective's. We did not find a single genuine exception.

Two oddities turned up, and neither breaks the rule. Distractability and perfectability appear in the British list, where the usual adjectives are distractible and perfectible. Both are rare minority spellings: Merriam-Webster lists distractable as a less common variant of distractible, and gives the noun of perfectible as perfectibility. Where an adjective itself has two accepted spellings, the noun just follows whichever you chose: collectible, or less commonly collectable, gives collectibility.

The same holds for the small adjustments the adjective makes. Words that drop a silent e before -able keep it dropped in the noun, so usable gives usability. Words that keep the e to hold a soft c or g keep it in the noun too: changeable gives changeability, and serviceable gives serviceability.

That is a rare thing in English spelling: a rule without exceptions worth memorizing. Its only weakness is that it moves the question back one step, from the noun to the adjective.

So the real question is the adjective

If you can spell available, you can spell availability. The difficult choice, as so often, is able or ible, and that has its own test, explained in full in able or ible: the rule that actually works. It works like this: take the ending off and see what is left. If it is a whole English word, the ending is almost always able. If it is a fragment, the ending is usually ible.

Run availability through it. Strip -ability and you are left with avail, a whole word, so the adjective is available and the noun is availability. Strip -ibility from responsibility and you are left with respons, which is not a word, so the ending is ible, as in responsible.

The exceptions that catch people in the adjectives catch them in the nouns, no more and no fewer. Accessible, flexible, convertible and deductible all take ible even though a whole word is left behind, and so their nouns are accessibility, flexibility, convertibility and deductibility. Learn the adjective and you have the noun for free.

A relative that says the vowel out loud

There is a rule we hold to on this blog: a related word helps you spell only if, in ordinary speech, it makes audible the exact letter you are unsure of. Most -ability words have no such relative. Nothing in everyday English says the a of availability or the i of responsibility out loud. For those, the adjective and the able or ible test are your only guides.

A smaller group does have one, and the reason is history. According to Merriam-Webster, the ending goes back to Latin -abilis and -ibilis, where the a or i was the vowel of the verb underneath. Many of those same Latin verbs also gave English a noun ending in -ation, and in -ation that vowel carries the stress.

So probable and probation come from the same Latin verb, probare, and in probation, pro-BAY-shun, the a is loud. Variable and variation both come from variare, and you can hear the a in vair-ee-AY-shun. The same goes for applicable and application, durable and duration, tolerable and toleration, and navigable and navigation. On the other side, audible and audition both come from audire, to hear, and audition stresses the i: aw-DISH-un.

One caution keeps this honest. The trick works only when both words really come from the same verb. Sensation has a loud a, but sensible is spelled with an i. Both go back to the Latin sensus, yet sensation was coined much later, in Medieval Latin, and its a was never the vowel of the verb behind sensible, which is sentire, to feel. Use the -ation word to confirm a spelling you have already worked out, never to decide one on its own.

The words that come up at work

Most people who search for this are not writing essays about Latin. They are writing job descriptions, project plans, contracts and support tickets, where -ability nouns are everywhere. Here are the ones that turn up most in working life, sorted by ending.

Run the adjectives through the test and nearly all of them behave. In the middle column, avail, account, scale, use, rely, profit, afford and sustain are whole words, and liable and capable are a-words that people rarely get wrong. In the right-hand column, respons, elig, compat, feas and vis are fragments, while access, deduct and flex are the known exceptions. If you write any of these daily, the table is worth a second look, because the error in a heading or a contract is the one everyone notices.

How to make it automatic

Knowing a rule and using it at typing speed are different skills. In a 1988 study, the literacy researcher Joanne Carlisle found that students in fourth, sixth and eighth grade could produce derived words like these aloud well before they could spell them. Spelling was harder, too, when the word changed more between its base and its derived form. Availability changes a great deal, gaining two syllables and moving its stress, so it is exactly the kind of word that needs practice and not just understanding.

The research on teaching word structure is encouraging. A 2010 review by Peter Bowers, John Kirby and Hélène Deacon, which pooled 22 studies of children, found that teaching about word parts benefits learners, on measures that included spelling as well as reading and vocabulary. Terezinha Nunes and Peter Bryant, in classroom work in London and Oxford described in their 2006 book Improving Literacy by Teaching Morphemes, found the same kind of benefit. Those studies were with children, but the mechanism they tested, seeing the parts so the whole is easier to spell, is the same one the adjective test asks you to use.

The practice itself is simple. Spell the adjective and its noun as a pair, from memory: available, availability; responsible, responsibility. Have someone say them, or play a recording, then write both without looking and check every letter. Henry Roediger and Jeffrey Karpicke showed in 2006 that recalling material builds much stronger memory than reading it again, even though rereading feels more productive. Do it today, tomorrow and again in a few days. Spelling.School works on the same principle: a recorded voice says each word, you type it, the app shows exactly which letter went wrong, and the word comes back for review on a spaced schedule.

What to do next

Write down the three -ability or -ibility words you use most at work. For each one, write the adjective beside it and run the able or ible test on the adjective, not the noun. If it is one of the exceptions, such as accessible or flexible, put a mark next to it. Then spell each pair from memory tomorrow morning, before you look.

If you want to see the wider set of words adults trip over, with a memory hook for each, the hundred words adults misspell most covers several of their adjectives, such as responsible and accessible. The five-word demo on the home page will say words aloud for you to type, with no sign-up. Or check the spelling of availability whenever it comes up, until you no longer need to.

Questions people ask

Is it availability or availibility?

The correct spelling is availability, with an a after the l: a-v-a-i-l-a-b-i-l-i-t-y. The noun is built from the adjective available, and a noun ending in -bility always keeps the vowel of its adjective. Availibility is a common misspelling because the stressed syllable of the word, BIL, has an i, and the a just before it is an unstressed vowel that sounds like any other.

How do you spell responsibility?

Responsibility is spelled r-e-s-p-o-n-s-i-b-i-l-i-t-y. It comes from responsible, which ends in -ible, so the noun ends in -ibility. People who also speak Spanish, French or Portuguese sometimes write responsability, because their own words for it, responsabilidad, responsabilité and responsabilidade, use an a.

Why is it possibility but probability?

The two nouns copy the vowels of their adjectives, possible and probable, and those come from different Latin words. Possible goes back to Latin possibilis, from posse, to be able, while probable goes back to probabilis, from probare, to test or prove. The related word probation, in which the a is stressed, is a handy reminder that probability has an a.

Is there a rule for ability and ibility?

Yes, and it is unusually reliable: an -ability or -ibility noun keeps the vowel of the adjective it comes from, so available gives availability and visible gives visibility. In a check of more than 400 such nouns in standard spell-check dictionaries, no genuine exception turned up. The only real decision is whether the adjective ends in -able or -ible.

How do you remember able or ible?

Take the ending off and look at what is left. If it is a complete English word, as with avail in available or accept in acceptable, the ending is almost always able. If it is only a fragment, as with poss in possible or vis in visible, the ending is usually ible. A short list of exceptions, including accessible, flexible and deductible, is worth learning by heart.

Is it accessibility or accessability?

The correct spelling is accessibility, with an i, because the adjective is accessible. Accessible is one of the well-known exceptions to the able or ible test, since access is a whole word but the ending is still ible. Once you know the adjective, the noun follows automatically, as it does for flexibility and deductibility.

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