The term derives from the Italian sonetto, a ‘little sound’ or ‘song’. Except for the curtal sonnet the ordinary sonnet consists of fourteen lines, usually in iambic pentameters with considerable variations in rhyme scheme. The three basic sonnet forms are: (a) the Petrarchan, which comprises an octave rhyming abbaabba and a sestet , rhyming edecde or cdedcd. or in any combination except a rhyming couplet ; (b) the Spenserian of three quatrains and a couplet, rhyming abab, bcbc, cdcd, ee; (c) the Shakespearean, again with three quatrains and a couplet, rhyming abab, eded, efef, gg.
The Italian form is the commonest. The octave develops one thought; there is then a ‘turn’ or volta, and the sestet grows out of the octave, varies it and completes it.
In the other two forms a different idea is expressed in each quatrain; each grows out of the one preceding it; and the argument, theme and dialect are concluded, ‘tied up’ in the binding end-couplet.
The Petrarchan sonnet probably developed from the Sicilian strambotto. It consisted of two quatrains to which were added two tercets . The earliest sonnets are attributed to Giacomo da Lentino (c. 1215-33) of the Sicilian School. But the form may have been invented by another poet at the court of the Emperor Frederick II in Sicily. At any rate, throughout the later Middle Ages, the form was used by all the Italian lyric poets, notably Guinicelli, Cavalcanti asnd Dante. They usually used it for love poetry and more particularly for that semi-Platonic and semi-religious devotion to the Lady or Donna which subsequently became a cliche of love pertry.