In the five stanzas following the first one which sets it up; the same six words must end the six lines, in a strictly prescribed variation of order. The variation is this: if we number the six words that end the first stanza's lines as 123456, these same words will switch places in the following sequences-- 615243, 364125, 532614, 451362, and 246531. The six words are then included within the lines of the concluding triplet (also called the envoy or tornada), again in a prescribed order: the first line containing 2 & 5, the second line containing 4 & 3, and the final line containing 1 & 6.
The 12th century Provencal troubadour Arnaut Daniel is credited with having invented the sestina form. Writers such as Dante, Philip Sidney, A. C. Swinburne, Rudyard Kipling, Ezra Pound, W. H. Auden, and Elizabeth Bishop are all noted for having written sestinas of some fame.
As an example of the way in which a sestina's end-words shift, below is a modern translation of the first two stanzas of a sestina by Dante Alighieri.