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Knowledgeable and likeable and other other such words place an e before the a in able. However other words like writable and receivable don't.

Why is that? Is there a rule behind this?

Also the word writable just seems plain wrong because I keep reading it as rit-able not right-able; the e would definitely help in this situation. On the other hand, I am able read receivable as intended...

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    "Likable" often doesn't have an e. – herisson Jan 17 '17 at 20:25
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    Basically, it's that way because it's that way. I vaguely recall seeing some "rules" for this 60-65 years ago, but as usual there was a bucket-load of exceptions. You are correct that the omitted "e" causes the rule for long/short vowel sounds to be violated, but likely different rule-makers were involved in the two cases. – Hot Licks Jan 17 '17 at 20:27
  • Normally, a verb that doubles its consonant before "-ed" or "-ing" also doubles it before "-able", so a hypothetical verb "rit, ritting, ritted" would be expected to correspond to "rittable", not "ritable". – herisson Jan 17 '17 at 20:28
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    Also see When to drop the 'e' when ending in -able? and Creating words with “-able” suffix; however, the duplicate suggested by sumelic has the most complete answer, in my opinion. – choster Jan 17 '17 at 20:46
  • ah, thanks, the answers are definitely on the other thread. – Govind Rai Jan 17 '17 at 20:50
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The trend is to drop the "e" before adding the suffix -able. The two terms you cite have both forms and only knowledgeable is more common than knowledgable, probably because it is an old coinage.

  • As the living suffix, -able is useful for coining new words, though we often have to ignore spell check when it comes to -able coinages. For example, our spell check disapproves of sanctionable, channelable, overthrowable, redoable, and torturable, but these are perfectly good words and do not require hyphenation.

  • To form an -able word, treat the verb as you do when making an -ing participle. For example, we make moving from move by dropping the e and adding -ing. So, to make move‘s -able adjective, we drop the e and add -able: movable.

(The Grammarist)

knowledgeable (adj.) also knowledgable, (Ngram):

  • c. 1600, "capable of being known, recognizable" (a 17c. sense now obsolete), from knowledge in its Middle English verbal sense + -able. The sense of "having knowledge, displaying mental capacity" is from 1829 and probably a new formation.

likeable (adj.) also likable, (Ngram):

  • 1730, a hybrid from like (v.) + -able. Related: Likeableness. Middle English had likeworthy (from Old English licwyrðe "agreeable, acceptable").
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If there were two t's in "writable" it would be pronounced "rit", like "written". There are lots of examples of this rule in English. Biting Bitten

Writing Written.

Kite Kitten

Lite Litter

Mite Mitten

and so on. Of course, English being what it is, you will come across exceptions to the rule

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