Most sonnets are written using iambic pentameter. That means each of the fourteen lines in a sonnet has 10 syllables that alternate between an unstressed syllable and a stressed syllable. Using iambic pentameter gives the poem a certain rhythm that makes it pleasing to hear when read aloud, but it can also become monotonous, so other types of meter are sometimes substituted. Typically this is done with a trochaic foot, which is a stressed syllable followed by an unstressed syllable, or by using a feminine rhyme to sneak in an extra syllable at the end of a line. Also, sometimes iambic hexameter or iambic tetrameter is used instead of iambic pentameter.
In terms of structure, Petrarchan sonnets consist of two parts: an octave and a sestet. The octave, the first eight lines, consists of two quatrains with "abba" rhyme scheme. The rhyme scheme of the sestet, the last six lines, can vary between "cde cde" and "cdc dcd." If you are not familiar with rhyme scheme, "abba" means the first line rhymes with the fourth line, and the second line rhymes with the third. In terms of theme in Petrarchan sonnets, the first eight lines of the poem are used to present the problem or ask a question and the last six lines either solve the problem or make the point the poem was trying to make. The transition between asking the question and solving the problem is called the turn; this occurs at line nine.
The Shakespearean sonnet consists of three quatrains, four line stanzas, and a couplet, which is two lines.. The rhyme scheme of the poem is "abab cdcd efef gg" and it is written in iambic pentameter. Like with the Petrarchan sonnet, there are two parts to the Shakespearean sonnet but the turn, sometimes called the Volta or change in action, is not located in a set position with the Shakespearean version. In some cases the turn begins with the first line of the third quatrain while in others the turn does not come until the rhyming couplet at the end.
Similar to the Shakespearean sonnet, Spenserian sonnets consist of three quatrains and a rhyming couplet. The rhyme scheme of a Spenserian sonnet interlocks the quatrains so it has this pattern: "abab bcbc cdcd ee." Though the language of Spenserian sonnets, with words like "yet" and "but," would make it seem the turn is placed on line nine, it is typically reserved for the couplet. Each of the quatrains briefly develops an individual idea, though each idea is closely related, and the couplet provides a commentary on previous ideas.