Originating in Europe, the word sonnet derives from the Occitan word "sonet" and the Italian "sonetto," both of which means "little song or sound." Here are the main differences between the Shakespearean and Petrarchan sonnet forms.
The Shakespearean style of sonnet usually contains 14 lines following a specific rhyme scheme and structure. Shakespeare's' sonnets all contain ten syllables per line and are written in iambic pentameter,
in which a pattern of an unemphasized syllable followed by an emphasized syllable is repeated five times. The rhyme scheme in a Shakespearean sonnet is a-b-a-b, c-d-c-d, e-f-e-f, g-g; the last two lines are a rhyming couplet.
Not all poets follow the specific metrical structure, since some sonnets contain 12 syllables per line.
Petrarchan sonnets, usually first credited to Giacomo da Lentini, were popularized by the Italian Petrarca (or Petrarch). They usually consist of two parts forming a
compact form of "argument". First, the octave (two quatrains), forms the "proposition" which describes a "problem", followed by a sestet (two tercets), which proposes a resolution. Typically, the ninth line creates what is called the "turn" or "volta" which acts to signal 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. In the sonnets of Giacomo da Lentini, the octave rhymed a-b-a-b, a-b-a-b; later, the a-b-b-a, a-b-b-a pattern became the standard for Italian sonnets. For the sestet there were two different possibilities, c-d-e-c-d-e and c-d-c-c-d-c. In time, other variants on this rhyming scheme were introduced such as c-d-c-d-c-d.