A sonnet is a poem in a specific form which originated in Italy; Giacomo da Lentini is credited with its invention.
The term sonnet is derived from the Italian word sonetto (from Old Provençal sonet a little poem, from son song, from Latin sonus a sound). By the thirteenth century it signified a poem of fourteen lines that follows a strict rhyme scheme and specific structure. Conventions associated with the sonnet have evolved over its history. Writers of sonnets are sometimes called "sonneteers", although the term can be used derisively.
The sonnet was created by Giacomo da Lentini, head of the Sicilian School under Emperor Frederick II.Guittone d'Arezzo rediscovered it and brought it to Tuscany where he adapted it to his language when he founded the Siculo-Tuscan School, or Guittonian school of poetry (1235–1294). He wrote almost 250 sonnets. Other Italian poets of the time, including Dante Alighieri (1265–1321) and Guido Cavalcanti (c. 1250–1300), wrote sonnets, but the most famous early sonneteer was Petrarca (known in English as Petrarch). Other fine examples were written by Michelangelo.
The structure of a typical Italian sonnet of the time included two parts that together formed a compact form of "argument". First, the octave (two quatrains), forms the "proposition", which describes a "problem", or "question", followed by a sestet (two tercets), which proposes a "resolution". Typically, the ninth line initiates what is called the "turn", or "volta", which signals the move from proposition to resolution. Even in sonnets that don't strictly follow the problem/resolution structure, the ninth line still often marks a "turn" by signaling a change in the tone, mood, or stance of the poem.