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So You wanna.com: So You Wanna Learn About Poetic Form?
Discusses poetic form and the basic unit of form, the stanza. Also defines many related terms including couplet, triplet (tercets), quatrain, sesten (sestina), rhyme royal, ottava rima, Spenserian stanza, sonnet, and villanelle.
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Bob's Byway: Glossary of Poetic Terms
Calling itself "unique," Bob's is easy to use, with cross-links throughout, phonetic pronunciation guides when necessary, and many examples and quotations. Click on the letter and scroll for the word.