Sestinas are a type of six-stanza verse, consisting of six stanzas of six lines each, with the same end words repeated in a specific pattern. The end words of the first stanza are repeated in a different order in each of the following stanzas, and the last stanza is a tercet, or three-line stanza, with the six end words appearing in the middle line. This form of poetry has been popular since the 13th century, and is still used today by poets to create intricate and complex works.