Welcome to what is probably the one spelling question spellers ask me more than any other. If you get a word that ends in what the dictionary shows as \əbəl\, do you spell it –able or –ible? For example, is it corrodable or corrodible? Acceptable or acceptible? Divisable or divisible? And once you figure out which of the spellings is correct…why is it correct? And how can we apply a pattern or a rule to other words for which we don’t know the spelling?
As it turns out, the language powers that be have a diabolical sense of humor. There’s a good reason this question is so frequently asked. There is no easy answer! I wish I could say “when such-and-such happens, it’s –able, and in all other cases it’s –ible.” But it’s not that simple. In fact, this is one of the trickiest orthographic conundrums in the English language, with rules that are rife with exceptions. So strap yourselves in. This will take a few posts to untangle.
Let’s start simply, though. If you are a statistician, you’ll enjoy this post, which will probably suffice for this question. As of today (September 12th, 2019), there are 2755 entries in Merriam Webster Unabridged Online that refer to –able as a suffix. But there are only 537 entries that refer to –ible as a suffix. Basically, this means that if you get an –able/-ible word, and you guess –able, you’ll have just shy of an 84% chance of getting the word right. Pretty good odds for an educated guess!
Now…I give the date of this statistic because the dictionary – and in particular, Merriam Webster Unabridged Online – is forever changing. Words are always being added. Some are being eliminated. Or perhaps they remain but end up tagged as obsolete or archaic. But really, the bulk of the English language is solid, so if this statistic changes, it won’t do so appreciably. This is gratifying to note. In essence, spellers fifty years from now will be able to rely on this rule as much as spellers did fifty (or even a hundred) years ago!