The Sestina

One of the most complex forms. Here is an overview of the form from poets.org

The sestina is a complex form that achieves its often spectacular effects through intricate repetition. The thirty-nine-line form is attributed to Arnaut Daniel, the Provencal troubadour of the twelfth century. The name "troubadour" likely comes from trobar, which means "to invent or compose verse." The troubadours sang their verses accompanied by music and were quite competitive, each trying to top the next in wit, as well as complexity and difficulty of style.

Courtly love often was the theme of the troubadours, and this emphasis continued as the sestina migrated to Italy, where Dante and Petrarch practiced the form with great reverence for Daniel, who, as Petrarch said, was "the first among all others, great master of love."

The sestina follows a strict pattern of the repetition of the initial six end-words of the first stanza through the remaining five six-line stanzas, culminating in a three-line envoi. The lines may be of any length, though in its initial incarnation, the sestina followed a syllabic restriction. The form is as follows, where each numeral indicates the stanza position and the letters represent end-words:

1. ABCDEF
2. FAEBDC
3. CFDABE
4. ECBFAD
5. DEACFB
6. BDFECA
7. (envoi) ECA or ACE

The envoi, sometimes known as the tornada, must also include the remaining three end-words, BDF, in the course of the three lines so that all six recurring words appear in the final three lines. In place of a rhyme scheme, the sestina relies on end-word repetition to effect a sort of rhyme.