Sestet

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:
Wordsworth and Milton are both remarkable for the dignity with which they conduct the downward wave of the sestet in their sonnet. The French sonneteers of the 16th century, with Ronsard at their head, preferred the softer sound of the arrangement aab ccb I. The German poets have usually wavered between the English and the Italian forms.
A sestet is also six lines of poetry forming a stanza or complete poem.
NOTE: The origin of the sonnet form has been traced to those poems by Giacomo di Lentino in Sicily. This work was prior to Francesco Petrarca's Italian sonnet form. The true original of the sonnet form is the Sicilian Sonnet (also in octave and sestet) rhyming abababab || cdecde or cdcdcd. It is generally believed that the first eight lines derive from the Sicilian form of the Stramboto.