I was talking with my friend Liz the other day and she mentioned that someone had asked on the Glen Mitchell show about whether there was any rhyme or reason behind why some words end in “able” (such as “comfortable”) while others end in “ible” (such as “horrible”). Before I get to the answer, though, here’s the scoop on the Glen Mitchell Show for readers who aren’t in Dallas — the show airs on Dallas’ local NPR affiliate and each Friday show features call-in questions which other audience members are invited to help answer. It’s a bit like Ask.MetaFilter but on the radio (HHOS).
Anyhow, you wouldn’t think that there’d be any reason behind such word endings, but there actually is (well, mostly).
If the root is not a complete word, add -ible. (aud + ible = audible)
If the root is a complete word, add -able. (accept + able = acceptable)
For example, “laugh” makes sense on its own, so that becomes “laughable”. On the other hand, “elig” can’t stand on its own, so that one’s “eligible”. Then again, English just isn’t allowed to be that easy ;), so there are some exceptions, such as “irritable” and “inevitable”.