A sonnet is a fixed poetic form with a structure traditionally consisting of fourteen lines adhering to a set rhyming scheme. The term derives from the Italian word sonetto (lit. 'little song', from the Latin word sonus, lit. 'sound'). Originating in 13th-century Sicily, the sonnet was in time taken up in many European-language areas, mainly to express romantic love at first, although eventually any subject was considered acceptable. Many formal variations were also introduced, including abandonment of the quatorzain limit – and even of rhyme altogether in modern times.

Giacomo da Lentini is credited with the sonnet's invention at the Court of Frederick II in the Sicilian city of Palermo. The Sicilian School of poets who surrounded Lentini then spread the form to the mainland. Those earliest sonnets no longer survive in the original Sicilian language, however, but only after being translated into Tuscan dialect. The form consisted of a pair of quatrains followed by a pair of tercets with the symmetrical rhyme scheme ${\displaystyle \mathrm {ABABABAB\,\,CDCDCD} }$, where the sense is carried forward in a new direction after the midway break.

Peter Dronke has commented that there was something intrinsic to its flexible form that contributed to the sonnet's survival far beyond its region of origin. William Baer suggests that the first eight lines of the earliest Sicilian sonnets are identical to the eight-line Sicilian folksong stanza known as the Strambotto. To this, da Lentini (or whoever else invented the form) added two tercets to the Strambotto in order to create the new 14-line sonnet form.

In contrast, Hassanally Ladha has argued that the Sicilian sonnet's structure and content drew upon Arabic poetry and cannot be explained as the "invention" of the Sicilian School of poets. Ladha notes that "in its Sicilian beginnings, the sonnet evinces literary and epistemological contact with the qasida", and emphasizes that the sonnet did not emerge simultaneously with its supposedly defining 14-line structure. "Tellingly, attempts to close off the sonnet from its Arabic predecessors depend upon a definition of the new lyric to which Giacomo's poetry does not conform: surviving in thirteenth-century recensions, his poems appear not in fourteen, but rather six lines, including four rows, each with two hemistiches and two 'tercets' each in a line extending over two rows." In Ladha's view, the sonnet emerges as the continuation of a broader tradition of love poetry throughout the Mediterranean world and relates to such other forms as the Sicilian strambotto, the Provençal canso, the Andalusi Arabic muwashshah and zajal, as well as the qasida.

Guittone d'Arezzo rediscovered the sonnet form and brought it to Tuscany, where he adapted it to Tuscan dialect when he founded the Siculo-Tuscan, or Guittonian school of poetry (1235–1294). He wrote almost 250 sonnets. Among the host of other Italian poets that followed, the sonnets of Dante Alighieri and Guido Cavalcanti stand out, but later the most famous and widely influential was Petrarch.

The structure of a typical Italian sonnet as it developed included two parts that together formed a compact form of "argument". First, the octave forms the "proposition", which describes a "problem" or "question", followed by a sestet (two tercets) that 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 do not 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.

Later, the ${\displaystyle \mathrm {ABBA\,\,ABBA} }$ pattern became the standard for Italian sonnets. For the sestet, there were two different possibilities: ${\displaystyle \mathrm {CDECDE} }$ and ${\displaystyle \mathrm {CDC\,\,CDC} }$. In time, other variants on this rhyming scheme were introduced, such as ${\displaystyle \mathrm {CDC\,\,DCD} }$ or ${\displaystyle \mathrm {CDE\,\,DCE} }$. Petrarch typically used an ${\displaystyle \mathrm {ABBA\,\,ABBA} }$ pattern for the octave, followed by either ${\displaystyle \mathrm {CDE\,\,CDE} }$ or ${\displaystyle \mathrm {CDC\,\,CDC} }$ rhymes in the sestet.

At the turn of the 14th century there arrive early examples of the sonnet sequence unified about a single theme. This is represented by Folgore da San Gimignano's series on the months of the year, followed by his sequence on the days of the week. At a slightly earlier date, Dante had published his La Vita Nuova, a narrative commentary in which appear sonnets and other lyrical forms centred on the poet's love for Beatrice. Most of the sonnets there are Petrarchan (here used as a purely stylistic term since Dante predated Petrarch). Chapter VII gives the sonnet "O voi che per la via", with two sestets (${\displaystyle \mathrm {AABAAB\,\,AABAAB} }$) and two quatrains (${\displaystyle \mathrm {CDDC\,\,CDDC} }$), and Ch. VIII, "Morte villana", with two sestets (${\displaystyle \mathrm {AABBBA\,\,AABBBA} }$) and two quatrains (${\displaystyle \mathrm {CDDC\,\,CDDC} }$). Petrarch followed in his footsteps later in the next century with the 366 sonnets of the Canzionere, which chronicle his life-long love for Laura.