I've never heard of them either. "cvc" apparently means a "consonant-vowel-consonant" pattern in a word, and "cvce" apparently means "consonant-vowel-consonant-with a final e" pattern. So cvc: gum wax cvce: wipe dune safe mule have vote race done tide wife note rule cape The others: lots, skin, drip, crop, crab, and which, don't have the cvc or cvce pattern.
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