Shakespeare's "Sonnet 18" is perhaps Shakespeare most famous sonnet and as such the most famous English language sonnet.

Much of its popularity comes from its perfect form. There is no what we would call enjambment—sentences that run into the next line. Each line is its own sentence and is written in what we call iambic pentameter. Iambic pentameter is a rhyme scheme popularised during Shakespeare's days and particularly adopted by sonnet writers.

The iambic pentameter has 10 syllables per line. These syllables are further divided into 5 iambic feet. Each iambic foot consists of one unstressed syllable followed by one stressed syllable.

Let's look at the first line of "Sonnet 18":

Shall I compare thee to a summer's day?

So you can see the first word "shall" is unstressed and the second word "day" is unstressed. This pattern continues through the first ten syllables and the remainder of the poem.

We can divide the sonnet into four further parts. We can divide the first twelve lines into 3 quatrains consisting of four lines each. A quatrain takes the rhyme scheme ABAB. That means that the first line will rhyme with the third line and the second line will rhyme with the fourth. We call the last two lines a couplet. Couplets rhyme with each other and usually aim to summarise the themes of the poem.

Shakespeare’s "Sonnet 18," certainly the best-known sonnet and one of the most famous poems in the English language, uses iambic pentameter as its meter. An iamb is a unit of two syllables, the first unstressed or short, and the second stressed or long. The scansion of a poem is sometimes indicated using marks above the words, or it can be written out using “di” for the unstressed syllable and “dum” for the long one. A line of iambic pentameter consists of five iambs in succession, and could therefore be written:

di dum di dum di dum di dum di dum

Looking at the sonnet, you will see that every line can be pronounced using this rhythm. To adhere to it with absolute precision is sometimes to mar the natural flow of the language. In the first line, for instance, the word “to” need not be stressed, though it falls on the second syllable of the third iamb. However, it does not seriously disrupt the meter to pronounce this word more lightly.

Elizabethan sonnets are not remarkable for their metrical regularity, and sonnet 18 is somewhat unusual in containing no major departures from the iambic pentameter form. Every line is exactly ten syllables, and the scansion is smooth and regular, as befits the subject of a beloved more clement than a summer’s day.

As a genre of poetry, sonnets are defined by having strict rules of structure, and as far as I am aware (at least within the English tradition), part of this entails a tendency towards iambic pentameter.

At the root of iambic meter is the iambic foot: this shapes the basic rhythm of the poem. An iambic foot is defined by the alternation of one unstressed syllable, joined with one stressed syllable. Iambic Pentameter contains five of these metric feet, totaling ten beats (by contrast, iambic tetrameter contains four metric feet, for eight beats total, whereas iambic trimeter contains three metric feet, containing six beats total). "Sonnet 18," as a close reading reveals, is written entirely in iambic pentameter. To read any of the fourteen lines that make up this poem, you'll find that same rhythm.

To take its initial line for example (stressed syllables are italicized, with each iamb separated within the text),

"Shall I / compare / thee to / a sum / mer's day?"

This is iambic pentameter.

The majority of Shakespeare’s works are written in a meter known as iambic pentameter. Sonnet 18 is no different. Iambic pentameter is a common meter, especially for works in and around Shakespeare’s time.

Iambic pentameter employs iambs. When speaking of meters, this is called a foot—the grouping and stressing of syllables. So an iamb is a grouping of two syllables, with the first unstressed and the second stressed, like the word “touché." In a pentameter, there are five (penta) feet, in this case iambs, per line.

An example of this type of meter would be a ten syllable phrase—“I am the man who saw you yesterday.” Stress each second syllable for “I am the man who saw you yes-ter-day”

This sonnet employs iambic pentameter. This means that each line contains ten syllables, divided evenly into five feet called iambs; each iamb contains two syllables: one unaccented (or unstressed) syllable followed by one accented (or stressed) syllable. Thus, the word iambic refers to the type of foot used overwhelmingly the poem. The word pentameter means that there are five of these feet on each line of the poem. For example, I will type in bold the syllables which should be stressed (or accented) below and separate feet with a "|" mark.

Shall I | com pare | thee to | a sum | mer's day
Thou art | more love | ly and | more tem | per ate
Rough winds | do shake | the dar | ling buds | of May
And sum | mer's lease | hath all | too short | a date

The meter of this sonnet is incredibly regular, and there are no deviations like extrametrical syllables or truncations or substitutions of one foot for another. The speaker is describing how beautiful and perfect his loved one is, and so it makes sense that he would use a perfectly regular meter to make the sound of the poem match its content.

If you are referring to William Shakespeare's Sonnet 18, it is written in iambic pentameter.  This means that there are five metrical feet per line (pentameter) and each foot contains an iamb which is identified by one unstressed syllable followed by one stressed syllable.

When marking meter on paper, unstressed syllables are usually marked with a backslash "/" and stressed syllables are usally marked with an "x."

Iambic pentameter was the most popular form of meter used during the Renaissance era and is also the most common form of meter used in sonnets.