Sestina: One of the more complicated of the French fixed forms of verse, the setina originated in medieval Provence. It has six unrhymed stanzas, in which the terminal words of each line are repeated in varying orders, followed by a tercet (a unit of three lines), which may include three of the terminal words or all six, used two to a line. In the diagram below, each letter represents the terminal word of a verse and each line a stanza:

a b c d e f
f a e b d c
c f d a b e
d e a c f b
b d f e c a
b d f e c a
e c a

(Beckson & Ganz, Literary Terms: A Dictionary. p. 255)