A sestina has seven stanzas, the first six of which are six lines long. The words that end each line of the first stanza are repeated in different orders to end the lines of the next five stanzas. Finally, the seventh stanza is three lines long, and each line incorporates two of the six established end-words.
The sestina originally adhered to more rules, such as a syllabic limit, but many poets have taken liberties with the form to drive their points home. "Railway Sestina", for instance, uses the same six end-sounds if not the same words.
My thanks to poet Kimberly Poitevin and reader Alison Dasho.