Let's get to the bottom of this whirlpool of a form. Some poetic forms are subtle and often flexible, like the sonnet. But the sestina has the handcuffs on just about as tight as they will go without ripping the hands right off your wrists.
Here are the rules: it's a poem made up of six stanzas of six lines each, followed by a final three-line stanza (a.k.a. tercet, a.k.a. envoi—which usually has a kind of P.S. feeling about it).
It uses the same six end words (words at the end of the line) in different order throughout the poem. For all the math geeks out there, here's the pattern:
Stanza 1: 123456
Stanza 2: 615243
Stanza 3: 364125
Stanza 4: 532614
Stanza 5: 451362
Stanza 6: 246531
Stanza 7 (the envoi): contains all six words.
The pattern is called retrograde cross. This means absolutely nothing to us, and probably nothing to you. It's about as confusing as it gets. If you try to write one of these, you can just write the pattern down and follow it in a fill in the blanks sort of way.
But here's the real question: why bother with all these rules in the first place. We mean, what does a poet have to gain by slapping some cuffs on themselves before they sit down to pen a poem?
Well for one thing, the repetition creates some pretty cool poetic effects. By forcing themselves to use the same word over and over again, the poet gets to imagine that word in new and exciting ways. But the repetition also has this nifty way of creating a sort of echo chamber in your mind. As readers, we get to meditate on these words and images, as the poet zeroes in on one moment in time—highlighting details we never could have imagined. Just check out how many different ways Bishop describes the almanac, or all the different things the grandmother does. It all happens so quickly, in the span of just a few minutes in this kitchen. But because of the repetition, we get to slow down and notice every little thing.