In Italian, a "sonnet" means a small song devoted to a solo or single idea. Though invented in Sicily sometime in the 1220s, the Italian sonnet was perfected by Frances Petrarch in the fourteenth century. Sir Thomas Wyatt brought the Petrarchan sonnet to England via translation in the sixteenth century and then reinvented the Italian form in English. Because Shakespeare mastered the English sonnet, the two terms are now loosely interchangeable.
What Petrarchan and Shakespearean sonnets (and indeed all sonnet forms) have in common is that the entire poem is structured as an extended metaphor. The sonnet builds up the central idea of the poem using different figures of speech. Often these metaphors and similes are strong and jarring which lends a sonnet its dramatic power. Since the sonnet originated in the court, it is particularly well-suited for expressions of love, both worldly and divine. The other concept common to both Petrarchan and Shakespearean forms is the twist in the central idea which occurs somewhere toward the end of the sonnet. The twist is a fresh take on the poem's conceit, often overturning it. Having looked at the similarities between the two forms, now let's move on to the differences.
A sonnet consists of 14 lines written in iambic pentameter. Typically, the Petrarchan sonnet is divided into two sections, with the first 8 lines (octave) following the rhyme scheme A-B-B-A-A-B-B-A and the next six lines (sextet) following either of the rhyme scheme C-D-C-D-C-D, C-D-E-E-D-E, or a close variant.
In the Petrarchan sonnet, the twist we spoke about earlier, the volta ("turn" in Italian), occurs around the ninth line and brings a freshness to the poem's central hypothesis. To illustrate this with an example, here's a twentieth-century version of the Petrarchan sonnet written by Edna St. Vincent Millay:
1. I, being born a woman and distressed (A)
2. By all the needs and notions of my kind, (B)
3. Am urged by your propinquity to find (B)
4. Your person fair, and feel a certain zest (A)
5. To bear your body's weight upon my breast: (A)
6. So subtly is the fume of life designed, (B)
7. To clarify the pulse and cloud the mind, (B)
8. And leave me once again undone, possessed. (A)
9. Think not for this, however, the poor treason (C)
10. Of my stout blood against my staggering brain, (D)
11. I shall remember you with love, or season (C)
12. My scorn with pity, —let me make it plain: (D)
13. I find this frenzy insufficient reason (C)
14. For conversation when we meet again (D)
Note the particular rhyme scheme of the sonnet as well as the volta (which begins with "Think not this..."). Through the turn, the woman's voice is making the bold point that the man may possess her body but never her brain and spirit. Thus she upturns the idea of her powerlessness put forward in the first line: "I, being born a woman and distressed."
The Shakespearean sonnet follows the Petrarchan tradition of the turn, but in form, it deviates significantly from its Italian counterpart. This no doubt has to do with the fact that it is far easier to rhyme in Italian than in English. In the Shakespearean sonnet, the 14 lines are divided into three quatrains (4-line stanzas) and an ending couplet. Significantly, the couplet was not used by Petrarch in his sonnets. The turn in the Shakespearean sonnet typically occurs around line 12 line and is amplified in the final couplet. The rhyme scheme usually is: A-B-A-B, C-D-C-D, E-F-E-F, and G-G.
We can see the deviation from the Petrarchan form in the following sonnet by Shakespeare (130), which incidentally is one of the most famous in his sonnet series:
1. My mistress' eyes are nothing like the sun; (A)
2. Coral is far more red than her lips' red; (B)
3. If snow be white, why then her breasts are dun; (A)
4. If hairs be wires, black wires grow on her head. (B)
5. I have seen roses damasked, red and white, (C)
6. But no such roses see I in her cheeks; (D)
7. And in some perfumes is there more delight (C)
8. Than in the breath that from my mistress reeks. (D)
9. I love to hear her speak, yet well I know (E)
10. That music hath a far more pleasing sound; (F)
11. I grant I never saw a goddess go; (E)
12. My mistress, when she walks, treads on the ground. (F)
13. And yet, by heaven, I think my love as rare (G)
14. As any she belied with false compare. (G)
Note the ending couplet typical to the English form, as well as the startling images: "If hairs be wires, black wires grow on her head." The turn here occurs after line 10, in the final couplet, when the poet claims that, despite all his beloved's human traits, his love for her is "rare" or precious—beyond any fake comparisons.