Spenser and Shakespeare both speak of love in their sonnets. They differ however in that Spenser's Amoretti sonnets were written for a woman who would not return his love until much time had past while Shakespeare wrote for loves who were won, though sometimes they had quarrels or obstacles. Spenser's chronicle unrequited love that finally has a triumphant ending while Shakespeare's chronicle moments in various requited (returned) loves.
In Spenser's Amoretti Sonnet 18, the theme is the hard heart of Spenser's beloved who loves him not and will not let his "long intreaty soften her hard hart." In Shakespeare's Sonnet 18, the theme is a comparison. The beauty of nature that fades under "Rough winds [that] shake the ... buds of May" and the sun's heat, that "too hot the eye of heaven shines" is compared to the beloved's beauty that, in opposition to nature's beauty, will not fade ("But thy eternal summer shall not fade") but instead will "in eternal lines to time" grow in beauty.
The structure of the sonnet was originated with the Latin poet Petrarch who structured sonnets as 14 lines comprised of an octave (8 lines) followed by a sestet (6 lines), with no end couplet. The Petrarchan rhyme scheme is abbaabba cdccdc, with concatenation in each part: aa cc. English poets varied this by changing the octave and sestet to three quatrains (4 lines each), producing 12 lines, and adding a rhyming couplet (2 lines) for the 13th and 14th lines. The standard rhyme scheme is abab cdcd efef gg, with distinct rhyme separations between each quatrain and the couplet thus eliminating concatenation. Shakespeare was not the originator of this English sonnet structure but is certainly the poet who immortalized it; the English structure is variously called the English sonnet or the Shakespearean sonnet.
Spenser developed a variation on the Petrarchan sonnet called the Spenserian sonnet. Spenser accomplished two thing with his sonnet structure: He (1) devised an interlocking rhyme scheme through which he (2) could develop either logical flow or opposition in the sonnet story. The Spenserian rhyme scheme is three quatrains and a rhyming couplet in ababbcbccdcd ee. The repetition of bb in the first portion and cc in the second links the rhymes together: the b rhyme carries from the first to the second quatrain through a repetition at the 4th and 5th lines while the c carries over from the second to the third quatrain through a repetition at the 8th and 9th lines.
This Spenserian/Petrarchan linking, or chaining, in the rhyme is called concatenation, built from the Latin root catena, which means chain. Just as the rhyme links, Spenser's stories can link (be chained together) through logical progression or they can logically oppose each other in thought. Sonnet 18 has logical opposition: It starts with the "rolling wheel" that can "teare" the "hardest steele," then progresses to the failure of his tears to affect the "hard hart" of his love who "turnes hir selfe to laughter" at his persistence. The steel is worn down by the rock in opposition to the "steele and flint" of her heart that "doth still remayne."
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