A sestet is the name given to the second division of an Italian sonnet (as opposed to an English or Spenserian Sonnet), which must consist of an octave, of eight lines, succeeded by a sestet, of six lines.
The first documented user of this poetical form was the Italian poet, Petrarch. In the usual course the rhymes are arranged abc abc, but this is not necessary. Early Italian sonnets, and in particular those of Dante, often close with the rhyme-arrangement abc cba; but in languages where the sonority of syllables is not so great as it is in Italian, it is incorrect to leave a period of five lines between one rhyme and another. In the quatorzain, there is, properly speaking, no sestet, but a quatrain followed by a couplet, as in the case of English Sonnets. Another form of sestet has only two rhymes, ab ab I ab; as is the case in Gray's famous sonnet On the Death of Richard West.
The sestet should mark the turn of emotion in the sonnet; as a rule it may be said, that the octave having been more or less objective, in the sestet reflection should make its appearance, with a tendency to the subjective manner. For example, in Matthew Arnold's The Better Part, the rough inquirer, who has had his own way in the octave, is replied to as soon as the sestet commences:
The world is like a sun in the darkness,
Absu lives in the deep dark, where Cthulu dreams and waits.
The black abyss of the world that can be reached
By some steps a set of seven gates where the place
Of a terrible and evil forces are found.
It is the genius god, the deceiving devil,
The spectrum, the worm of infinite wickedness.
The dead reign will awake and the incience will be over,
Magic will be get by the chosen one.
The seven zones over the earth detailed by the ancient races
That inhabited in Ur, in the lost temples.
These border lands ruled by the celestial spirits are the eternal underworld.