How does the use of rhythm and rhyme used in sonnet 18 assist the reader's understanding of the poem?Shakespeare's Sonnet 18

Written in a sequence and addressed to the object of the
poet's passion, Shakespeare's sonnets follow the English sonnet form with its three
four-line quatrains plus a concluding two-line couplet are used to trace the development
of this relationship of the poet and his lover.  More commonly known as the
Shakespearean sonnet, the rhyme scheme [regular pattern of rhyming words] is as
follows:abab cdcd efef gg.  Each of the three quatrains explores a
different variation of the theme and the rhyming couplet presents a summarizing or
concluding statement on this theme. For instance, the first quatrain of Sonnet
18 presents the rhetorical question of comparing the lover to a summer's
day.  The poet declares that the lover is "more lovely and more temperate," and he
demonstrates the truth of his statement.  In what is known as a
volta, or turn in thought, the second quatrain explains how Nature
is sometimes too severe and beauty is destroyed.  Then, in the third quatrain, the poet
continues his argument that the object of his passion will possess eternal beauty
because the sonnet itself will attest to her beautyl: "But thy eternal summer shall not
fade," the poet declares as the lover will live on in his verse.  Finally, the rhyming
couplet concludes by reiterating the theme that the sonnet itself will render the love
eternal. Thus, the rhyme scheme controls the thematic development of the
sonnet.

The meter of the Shakespearean sonnet, or its
rhythmical pattern is iambic pentameter.  That is, there is a foot with one unstressed
syllable follwed by one stressed syllable as in the word afraid
[a is unstressed, fraid is stressed].  Five of
these feet makes the pentameter.  For instance, "Shall I compare thee to a summer's
day?" which is the first line, exhibits this iambic pentameter.  (Using the elevated
short u and the slash mark (/), the reader can mark off the unstressed and stressed
syllables that occur in pairs five times.) This iambic pentameter follows naturally the
English emphasis upon words and syllables as well as the natural rise and fall of
English-speaking voices, thus lending reality and sincerity to the message of the
poem.