The sestina is a poetic form invented by the twelfth-century troubadour Arnaud Daniel. It is composed of six stanzas of six lines each, followed by a three-line envoi. Each stanza must use the same six words to end each of its six lines. The ordering of these end-words within a stanza changes according to a fixed pattern. The final three lines must also employ these six words, usually two per line.
The ordering pattern is as follows:
The first line of a stanza uses the same end-word as the last (sixth) line of the previous stanza.
The second line uses the same end-word as the first line of the previous stanza.
The third line uses the same end-word as the fifth line of the previous stanza.
The fourth line uses the same end-word as the second line of the previous stanza.
The fifth line uses the same end-word as the fourth line of the previous stanza.
The sixth line uses the same end-word as the third line of the previous stanza.
Modern sestinas often use iambic pentameter and occasionally leave off the envoi.
The Ode Less Travelled, Stephen Fry, Hutchinson (2005), ISBN 0 09 179661 X
The Making of a Poem, Mark Strand and Eavan Boland, Norton (2001), ISBN 0-393-32178-9