The octave in an Italian sonnet follows the rhyme scheme, ABBA, ABBA, where each A, representing the final word in a line, rhymes with every other A, and each B rhymes with every other B, and so on. The rhyme scheme of the sestet in an Italian sonnet varies; a common one is CDE, CDE. A Shakespearian sonnet uses the following rhyme scheme: ABAB, CDCD, EFEF, GG. The Shakespearian sonnet, with its distinctive rhyme scheme, is perhaps more suitable for poets writing in English, where there are far fewer rhymes than there are in Italian. There are many variations on these two rhyme schemes, but these are the most common and provide a point of reference.
Meter
Sonnets in English are usually written in iambic pentameter. "Iambic" refers to the type of foot, or rhythmic unit, used (in this case, the iamb), and "pentameter" refers to the number of feet in each line (in this case, five). An iamb is simply an unstressed syllable followed by a stressed syllable, and iambic pentameter is simply a line five iambs long. The word "confirm," for example, is an iamb. And just as we stress a particular syllable within a word, we also stress certain words within a sentence. Consider this line of iambic pentameter by Sidney: "And this I swear by blackest brook of hell." Listen to yourself saying it out loud and try to hear where the accents fall.
Not every line in a sonnet has to be perfectly iambic. In fact, it’s a good idea to vary the meter a little so that it doesn’t sound sing-songy or monotonous. A good way to do this is to switch the syllables around, so that the stressed syllable comes before the unstressed syllable. This type of foot is called a trochee. It is common to find trochees in sonnets, especially at the beginning of lines. This line of Surrey’s, for example, begins with a trochee: "Love, that doth reign and live within my thought." You can also add an unstressed syllable here and there to change things up, but, generally, there should always be five stressed syllables per line.
A caesura, or pause in a line, and enjambment, or not pausing syntactically at the end of a line, are other ways to vary the rhythm of the poem. A caesura can slow down the rhythm of a poem, and enjambment can be used to speed it up.