During the Renaissance, English poets adapted the Petrarchan sonnet, originated by Italian poet Petrarch, to suit English tastes, language, and ideas. There were several changes from the original Petrarchan model in structure and in expression of ideas. Petrarch, or Francesco Petrarca, lived from 1304 to 1374. English poets Wyatt and the Earl of Surry developed the Renaissance English sonnet in the early 1500s, roughly two hundred years later. Spenser modified Surry’s sonnet form and his form was in turn modified by poets like Drayton who perfected the English form later made famous by Shakespeare.
Petrarch developed his form to express two contrastive ideas that end by revealing the tension between them or submitting to a resolution of the tension. For example, in Petrarch's sonnet that begins, "She ruled in beauty o'er this heart of mine," Petrarch contrasts the qualities of his deceased beloved at her death (“blessings must resign”) to his suffering ("crushed with care") and the philosophical realization of what death is: “Assuredly but dust and shade we are, ….”
The English sonnet added a broader base for contrast so that English sonnets often progress through three distinct contrastive stages that end in a final ephiphanic resolution. For example, in Shakespeare's Sonnet 18, the speaker answers his question for 8 lines in the negative saying why he will not compare his love "to a summer's day." In lines 5 through 8, a minor contrast turns from a discussion of nature to a metaphorical philosophical consideration of the cause of fading beauty: "By chance or nature's changing course untrimm'd."
At line 9, Shakespeare brings in the major contrast element by turning to consideration of the metaphorical beauty of the "summer's day" of his beloved. He calls her beauty "thy eternal summer" and says it "shall not fade." This leads to the turn to the contrasting epiphanic resolution in the couplet that states his beloved shall live eternally, "So long as men can breathe or eyes can see," in the sonnet he writes in praise.
While Petrarchan sonnets have one major contrastive element with a resolution to the contrastive tension, English sonnets may have two, with a contrastive resolution, often in the form of an epiphany. This change developed out of the change to Petrarch's structure. Petrarch wrote sonnets of an octave (or octet) followed by a sestet [8 lines followed by 6 lines]. The turn to the contrasting comparison comes at line 9; the turn is called the volta. The fixed rhyme scheme of the octave is a b b a a b b a. It was followed by a rhyme schemes in the sestet which may be any of these:
c d c d c d
c d d c d c
c d e c d e
c d e c e d
c d c e d c
These rhyme schemes created a tight formal and ideological structure with the resolution lines being an integrated part of the sestet.
The English sonnet structure changed to three quatrains with rhyming pairs in each: abab cdcd efef. The quatrains are followed by a couplet: gg. The couplet separates the resolution from the quatrains thus facilitating the epiphanic resolution. The three sets of rhyming pairs separate the three quatrains from each other so the English sonnet may have two voltas, turns in contrastive thought, at lines 5 and 9.