A Petrarchan (or Italian) sonnet is constructed in the following way:
This particular sonnet is divided into two sections and contains two distinctively different rhyming patterns. The first section is called the octave because it consists of eight lines ("oct" is Latin for eight).
These eight lines have the rhyming pattern:
-a b b a a b b a.
The second section of the Petrarchan sonnet is called the sestet (consists of six lines-Latin for sixth). These lines do not rhyme with the preceding octave; instead, they contain a new rhyme scheme using the one of the following patterns:
The change in the rhyming pattern announces a change in the "plot" or subject matter of the poem. This change is notated by the "volta" in the ninth line of the sonnet.
The Shakespearean Sonnet (or the English Sonnet) is unlike the Petrarchan Sonnet in regards to the "plot"/meaning of the poem. In the Shakespearean Sonnet, the plot remains the same throughout the entire poem with each stanza building on the content which supports and defines the main idea of the poem- all stanzas are linked closely. The plot does not contain a major shift in subject matter. Both types of sonnets include the use of fourteen lines.
The Shakesperean Sonnet is constructed in the following way:
This sonnet consists of three quatrains (a stanza which consists of four lines and a couplet (a two line stanza). The rhyme scheme of this sonnet is the following:
A volta does exist within the Shakespearean Sonnet, but typically can appear at any point throughout the poem.