The Sestina follows a strict pattern: a specific repetition of the six end-words of the first stanza, as the end words of the five remaining six-line stanzas -- the six stanzas culminating in a three-line envoi. (See the formula below this explanation.) The lines may be of any length, though in its initial incarnation, the Sestina followed a syllabic restriction. In the contemporary Sestina, the use of occasional slant rhyme/half-rhyme end words is acceptable. (Slant rhyme, also known as half-rhyme or imperfect rhyme, refers to words that almost rhyme or appear to the eye to do so.) Regarding my Sestina "A Changing Heart," I took such liberties.
Here is the prescribed form for the "Sestina." (Each number represents the last word of each of the 6 lines of the 6 stanzas):
1st stanza 1 2 3 4 5 6
2nd stanza 6 1 5 2 4 3
3rd stanza 3 6 4 1 2 5
4th stanza 5 3 2 6 1 4
5th stanza 4 5 1 3 6 2
6th stanza 2 4 6 5 3 1
Envoi (the last 3 lines -- the last 2 words):
2-5
4-3
6-1
(that is the most common one I saw on line) (I also saw)
6-2
1-4
5-3