A sestina is a form that uses six six-line stanzas, each using the same six words at the end of its lines in different orders, followed by an envoi of three lines using two of those words to each line. They tend to be written in iambic pentameter, and without rhyme. Later sestinas sometimes allow homophones - such as 'hare' and 'hair' - for the repeat words, or even looser interpretations.
The order of repetition follows a spiral: after the first, each stanza begins with a line that ends in the last word of the last line of the stanza before, then one that ends in the last word of the previous stanza's first line; then second-from-last, then second; then third-from-last, then third. (Or, by way of a diagram, if the end words of one stanza are symbolised abcdef, the next stanza's end words would be in the order faebdc, the next cfdabe, and so on.) By the end of the sixth stanza, every word has been in every position within a stanza. The distribution of repeat-words in the envoi varies, but it is usual to demand two to a line with one at the end of the line.
Like a villanelle, examples make what might seem a dry set of rules into a living poem. Rudyard Kipling's 'Sestina of the Tramp Royal', Ezra Pound's 'Sestina: Altaforte' and Elizabeth Bishop's 'Sestina' are further well-known examples, and John Ashbery is the author of several, including one featuring Popeye. Within the Archive, Seamus Heaney's 'Two Lorries' is a good example, with the form being used to build the correspondences between the memories of a childhood, the later explosion, and a vision of death.