The sestina (less commonly, though more correctly, sextain) is a wondrous strange beast, the brainchild of a twelfth-century Provençal troubador. It doesn't use rhyme; instead, it has six keywords essential to the poem's structure. The poem's 39 lines - six 6-line stanzas followed by a 3-line envoi or tornada - all end with one of the keywords; in the tornada, there are two keywords in each line, one of them at the end and the other somewhere in the middle. It may all begin to make sense if we try an example.
stanza 1: 123456
stanza 2: 615243
stanza 3: 364125
stanza 4: 532614
stanza 5: 451362
stanza 6: 246531
This is the prescribed order for a sestina - at least, for an unrhymed one. (Yes, there are rhymed ones too. This is a variation dealt with later.) No deviation from this order is tolerated.
However, there are several different possible orders for the keywords in the tornada ("tornada schemes").
The popular schemes are 12/34/56, 14/25/36, 25/43/61 and 65/24/31. Pretty well anything goes, really.
You'll notice that each keyword appears once in the first line of a stanza, once in the second line of a stanza, and so on. You may also notice that the permutations of the keywords follow a regular pattern. It's all a bit like bell-ringing. Or mathematical group theory, for that matter.
At 39 lines, the sestina is eligible for poetry competitions with a 40-line limit. (Perhaps they used to have a lot of those in Provence.)
Pasted from <http://allpoetry.com/list/539645-Sestina>
The Rhymed Sestina
The most important recognised sestina variant is the rhymed sestina, which was devised by Swinburne. Here keywords 1, 3 and 5 rhyme with each other, as do keywords 2, 4 and 6. The permutations are revised so that every stanza has the same rhyming scheme ababab. In terms of the keywords, the revised structure is:
stanza 1: 123456
stanza 2: 614325
stanza 3: 561432
stanza 4: 256143
stanza 5: 321654
stanza 6: 432561
This revision of the structure leaves the rhymed sestina relatively unappealing to group theorists or (presumably) bell-ringers.
NB This structure is only "correct" if your keywords rhyme as stipulated above. For an unrhymed sestina, you must use the structure described on the main sestina page. In an ideal world this would not matter, but be warned - I once wrote an unrhymed sestina The Suicidal Goldfish for which I mistakenly used the structure of a rhymed sestina. That mistake was enough for one of the UK's better poetry magazines to decide not to publish it after all!