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Themes
Immortality
Although it is likely that Shakespeare himself did not arrange his 154 sonnets into groups, critics have come to recognize patterns or stages of their sequence. They have noticed, for example, that one dominant theme in Sonnets 1-17 is immortality through procreation. In the first seventeen sonnets a young man is urged to marry and have children. This is a very conventional theme for Elizabethan sonnets, but in "Sonnet 18," Shakespeare advocates seeking immortality, through poetry rather than through procreation: he wants to immortalize the object of his affection by creating a work of art that will last forever.
"Sonnet 18" is structured as an argumentative monologue delivered in response to the question- "Shall I compare thee to a Summer's day?" - posed in the first line. The speaker answers the question in the negative, suggesting that the object of his affection is "more lovely and more temperate" than a mere summer's day. Though summer days are pleasant, they are neither perfect nor everlasting. Their finiteness and propensity for bad weather make them, the speaker argues, a poor comparison with the object of his affection.
In the third quatrain (four-line stanza) the speaker refers to the object of his affection as an "eternal summer," whose loveliness and temperance are obviously more enduring than a summer's day. The "eternal lines" mentioned in line twelve, then, not only refer to the poetic lines of the sonnet, but also to the shape and beauty of the beloved. In the sonnet's couplet (pair of rhyming lines that concludes the poem), the speaker contends that because poetry is immortal, so, too, can his beloved's beauty remain immortal when preserved in verse: "So long as men can breathe, or eyes can see, / So long lives this, and this gives life to thee.''
Beauty / Aesthetics
In "Sonnet 18" Shakespeare closely relates the theme of beauty with the theme of immortality. The speaker's main contention, for example, explaining why the object of his affection is not comparable to a summer's day, revolves around the idea that his beloved is indeed everlastingly beautiful: "Thou art more lovely and temperate: ... / So long as men can breathe, or eyes can see, / So long lives this, and this gives life to thee." Comparing both the love the speaker feels, and the eternal beauty that his love possesses to a summer's day, then, is simply inadequate.
In the last two lines of the second quatrain, the speaker maintains that in the physical world, nature dictates that everything, even beauty, slowly decays. In the third quatrain, however, the speaker stops comparing his love with a summer's day, and instead describes the extent of his beloved's beauty: "But thy eternal Summer shall not fade / Nor lose possession of that fair thou ow'st; / Nor shall Death brag thou wander'st in his shade...." The speaker asserts that his beloved possesses a beauty so deep and enduring that it cannot be adversely affected by time and age. This beauty can even conquer death as long as there are people to read the lines of this poem.
Style
The sonnet (from the Italian sonnetto, or little song) owes much of its long-standing popularity to Petrarch. By the mid-sixteenth century, this fixed poetic form was adopted by the English, who borrowed the fourteen-line pattern and many of Petrarch's literary conventions. However, English writers did alter the rhyme scheme to allow for more variety in rhyming words: while an Italian sonnet might rhyme abba, abba, ede, ded, an English or Shakespearean sonnet rhymes abab, eded, efef, gg.
In all but three of Shakespeare's 154 sonnets ("Sonnet 99," "Sonnet 126," and "Sonnet 145"), the first three groups of four lines each are known as quatrains, and the last two lines are recognized as a couplet, lire three breaks between the quatrains and the couplet serve as convenient places where the writer's train of thought can take a different direction. In "Sonnet 18," a change in the course of the argument is marked by the word "but" at the beginning of the third quatrain. The final couplet does not simply affirm or contradict the speaker's main idea, but extends it: the beloved is indeed everlastingly young and beautiful, but only if the sonnet lives on.
The rhythm employed in "Sonnet 18" is known as iambic pentameter. Iambic meter, the most familiar rhythm in the English language, is simply the succession of alternately stressed syllables, in which the first is unstressed and the second is stressed. The use of "penta" (meaning five) before "meter" means that there are five iambs per line.
Stresses embody meanings. Therefore, when Shakespeare breaks from the iambic meter and has two or more stresses fall side by side, he not only adds variety but emphasis to certain lines. "Rough winds" (line 3), "too hot" (line 5), and "Death brag" (line 11) are examples of spondees, because they are comprised of two accented and consecutive syllables. The change in the regularity of the rhythm adds force to the first two descriptions, and calls attention to the specter of Death in line 11.