Possible Duplicate: When to drop the 'e' when ending in -able? Both are correct for these words: sizable, sizeable sharable, shareable takable, takeable But these words are incorrect: storeable (storable) makeable (makable) tasteable (tastable) minglable (mingleable) acknowledgable (acknowledgeable) managable (manageable) Are there rules for this? The case for keeping the 'e' seems to be that the 'ga' in 'acknowledgable' would be read as 'gall' rather than 'germ', but 'minglable' shouldn't cause that problem. I'm also not sure why we couldn't keep the 'e' in words like 'storable' or 'makable'.
Output: The general rule is that you drop the e, provided that it's actually a silent one rather than forming part of an ee digraph as with agreeable, decreeable, disagreeable, foreseeable, and seeable. You only otherwise have to keep a final silent e when adding able if it would (1) either change the pronunciation, or (2) it would cause a collision with an existing word that already means something else altogether different. The first case, pronunciation shift, is for things like raceable or purgeable, where losing the e would make the ce or ge switch from the 'soft' version to the 'hard' version due to the resulting g or c now being followed by an a rather than by an e, and cable and gable are pronounced quite differently. The second case, a collision with an existing word, occurs with such pairs as pareable / parable, placeable / placable, probeable / probable, and spareable / sparable. Notice how placeable also has the first factor in play, as too would singeable / singable. Beyond that, there is some slight variation in acceptable forms, such as both microwaveable / microwavable being attested. Normally however, you lose the e. The OED2 attests 1555 words than end in able but not in eable, while only 123 that do actually end in eable. That's a ratio of nearly 13 to 1 in favor of more able words without a preceding e. Not excluding duplicates with alternate spellings, here are the eable words from the OED2 plus quarterly supplements since 2000: acknowledgeable decreeable haveable mingleable salvageable acreable defaceable hireable mortgageable scribbleable activeable delineable holeable nameable seeable adduceable derangeable hopeable negligeable serviceable advanceable disagreeable imageable noticeable shakeable agreeable dischargeable immalleable packageable shapeable allegeable discourageable impermeable pareable shareable arrangeable displaceable inassuageable passageable sizeable balanceable divorceable indulgeable peaceable skateable batheable dyeable ineffaceable permeable sliceable biteable effaceable influenceable pierceable spareable blameable embraceable interchangeable placeable stageable bounceable enforceable irremeable pledgeable strangleable bridgeable exchangeable irreplaceable policeable swindleable carriageable experienceable irretraceable poseable takeable challengeable eyeable judgeable probeable tallageable chanceable fleeceable knowledgeable pronounceable tameable changeable foreseeable likeable purgeable tasteable chargeable forgeable liveable raceable throttleable chaseable garbleable lodgeable rateable traceable chooseable gaugeable malleable reable vengeable commenceable giveable manageable rebukeable voyageable cuddleable gorgeable marriageable replaceable whistleable damageable grazeable microwaveable ropeable danceable hateable mineable saleable As you see, there is also some tendancy to preserve the final e if in the original it's there to make a so-called 'long' vowel long. However, this doesn't happen consistently enough to make a rule for them. However, for those cases there are a lot of alternate 'sans'-e forms for those, like ropeable / ropable. Perhaps a more interesting question and certainly one with a more complicated answer is how to know which words instead take ible forms. There are 275 of these in the OED2, more than twice as many as eable words. There are actual rules there, although they are a bit complicated for some people. That one I'll leave for when it actually gets asked. :)