“What is the name of the poetic form that uses six six-line stanzas and a final three-line envoi?” The form is a sestina. Its defining feature is the repetition and rearrangement of six end-words across six stanzas.
A traditional sestina contains six stanzas of six lines each, followed by a shorter concluding stanza called a tornada or envoi. The six end-words from the first stanza recur in a prescribed order rather than being replaced by ordinary rhyme. This creates a tightly patterned poem driven by repetition, variation, and shifting meaning.
The form is generally credited to the 12th-century Occitan troubadour Arnaut Daniel. It later spread into Italian, French, English, and other literary traditions. English-language poets such as Elizabeth Bishop and W. H. Auden have written well-known sestinas.
The term is easy to confuse with other named forms. A sonnet usually has 14 lines, a villanelle uses recurring refrains and two rhyme sounds, and terza rima is built from interlocking three-line stanzas. A sestina is identified chiefly by its six repeated end-words and rotational pattern.