All sonnets are made up of 14 lines divided into sections. Each line has the same number of syllables -- meaning that when the poem is read aloud, the lines are of equal length. The sound of the last syllable of each line is chosen to produce a rhyming pattern, the specifics of which depends on the style of sonnet. In every sonnet, there is a shift, or turn, where a question is posed and answered, or where the emotion of the poem shifts.
The Italian sonnet is made up of an eight-line section followed by a six-line section, called an octave and sestet, respectively. Academics use lower-case letters to describe the rhyming pattern of a poem. The lines that finish with the same letter finish with the same sound. The octave of the Italian sonnet follows an "abbaabba" rhyming pattern. This means the 1st, 4th, 5th and 8th lines of the octave, or the "a" lines, will finish with the same sound. The 2nd, 3rd, 6th and 7th lines will also end with the same sound, but it will be a different sound than that heard in the remainder of the octave. The sestet rhyming pattern is usually either "cdecde" or "cdcdcd."
Shakespeare used the English sonnet style. This form is comprised of four sections, three of which are quatrains of four lines each. The last section is a couplet made of two lines. The rhyming scheme is "abab cdcd efef gg." The turn in the English sonnet comes before the last section.
The Spenserian sonnet -- so named because of its use by the poet, Edmund Spenser -- varies from the English sonnet in carrying a rhyme sound across sections. The scheme of a Spenserian sonnet is "abab bcbc cdcd ee." The Envelope sonnet is similar to the Italian sonnet, except that the second half of the octave uses a different rhyme from the first half. Therefore, the scheme is "abba cddc" for the octave, and either "efgefg" or "efefef" for the sestet.