If you, like me, took days of research trying to understand sestinas, you might find this useful.

A sestina (also, sextina, sestine, or sextain) is a highly structured poem consisting of six six-line stanzas followed by a tercet (called its envoy or tornada), for a total of thirty-nine lines. The same set of six words ends the lines of each of the six-line stanzas, but in a different order each time; if we number the first stanza’s lines 123456, then the words ending the second stanza’s lines appear in the order 615243, then 364125, then 532614, then 451362, and finally 246531.

First stanza, ..1 ..2 ..3 ..4 ..5 ..6
Second stanza, ..6 ..1 ..5 .. 2 ..4 ..3
Third stanza, ..3 ..6 ..4 ..1 ..2 ..5
Fourth stanza, ..5 ..3 ..2 ..6 ..1 ..4
Fifth stanza, ..4 ..5 ..1 ..3 ..6 ..2
Sixth stanza, ..2 ..4 ..6 ..5 ..3 ..1

Concluding tercet:

(line) ..2, ( line) ..5
(line) ..4, (line )..3
(line) ..6, (line) ..1

This organization is referred to as retrogradatio cruciata (“retrograde cross”). These six words then appear in the tercet as well, with the tercet’s first line usually containing 1 and 2, its second 3 and 4, and its third 5 and 6 (but other versions exist, described below). English sestinas are usually written in iambic pentameter or another decasyllabic meter.

An alternate form exists using a couplet, instead of a tercet, with the word orders 123 and 456 or 135 and 246.

The sestina was invented in the late 12th century by the Provençal troubadour Arnaut Daniel. Elements of it were quickly imitated by other troubadours, such as Guilhem Peire Cazals de Caortz.

Example I wrote:

dawn brings with it the smell of rain
streaks of yellow light the city
dewdrops lift from their sea of green
snow melts from its fortress of stone
dawn in the call of the mind
as an age rests on the streets forever.

but what makes eternity last forever
entombed in thunder along with rain
lost in the byways of the mind
crisscrossing the avenues of the city
bored holes within their concrete and stone
tufts of moss break grey with green.

dawn swiftly covers ruins with green
that remain a broken monument forever
even lower than plaques carved into stone
eroded now by the torrential rain
blurry memories of a forgotten city
that serve existence only in men’s minds.

what calls whence and wherefore, the mind
it dwells in its haven of blue and green
tripping, soaring above its dream city
will it cease to fly forever
should it encounter the rain
and lose itself in the solitude of stone?

dawn brings illumination to stone
towers that rise in the chasms of the mind
grey clouds gather to fling down the rain
covering and nourishing  hidden green
it pours down joy and tears, forever
upon the makeshift name of “city”.

soft yellow melts the cold, merciless city,
with its buildings in quiet, blissful stone
they resolve to stand forever
fragments of an unyielding mind
dawn brings the promise of green
and relief from the enduring rain.

if there was a city, if there was such a mind,
that still sought stone consumed by green
let it rust in peace forever, beneath the algid rain.

-Azuire//lastfactor&c.