What is a Sestina?
A sestina is an odd, usually unrhymed, form of French poetry that has been around since the 12th century which is composed of six six line stanzas and an ending three line stanza. What is especially noteworthy about this form is that it demands that an certain end word pattern be followed throughout the poem - for example, if you designate the end words of the first stanza with the numbers 1,2,3,4,5 and 6, the second stanza uses the same end words but in the combination 6,1,5,2,4,3 and so on. The complete word patterns used in the six stanzas are as follows:
There are however, different ways of composing the last three line stanza (called an envoy or tornada). Some say that the fist line should contain 1&2, the second 3&4 and the last 5&6, while others argue that it should be 1&4, 2&5, 3&6. There are several variations to this with the basic message being: if they're all there, it's all good.
Usually, for a sestina writter that's a challenge all by itself, but if you're looking to make things harder you can go for the even more difficult traditional form which says that each line is supposed to be in iambic pentameter. That's incredibly difficult; and a real challenge.
Sestina History
Sestinas have been around since the 16th century and its invention has oftenbeen attributed to the poet trabadour Arnaut Daniel. It fell into disuse until the 16th centory, when it was revived by Ferdinand, Comte de Gramount who wrote a large number of sestinas.