Form in poetry refers to its structure, including the number of lines, rhythm, meter and rhyme. At its most basic, a sonnet is a 14-line poem in iambic pentameter with variable rhyme schemes. Those 14 lines can be broken up into stanzas of various lengths, such as an octave, a stanza of eight lines, or a quatrain, a four-line stanza. The form of a sonnet determines its type.
Petrarchan, also called Italian, sonnets are divided into two parts – the octave and the sestet. The octave is an eight-line stanza with the rhyme scheme ABBAABBA. The sestet is a six-line stanza that can have various rhyme schemes, most often using CDCDCD or CDECDE, called the Sicilian or Italian sestet, respectively. The sestet could also employ CDDCDC, CDECED or CDCEDC rhyme schemes. The octave tends to ask a question or communicate tension, and the sestet resolves the problem or provides relief.
The English or Shakespearean sonnet contains three quatrains and ends with an heroic couplet, or two rhymed lines in iambic pentameter. This sonnet follows the rhyme scheme ABAB CDCD EFEF GG. The quatrains each develop a specific idea, although all subjects in the quatrains are related. The turn in the poem, also called the volta, comes around line 13, which puts great emphasis on the couplet.
Other Sonnet Styles
While the Italian and English sonnet forms are the most common, other styles exist. The Spenserian sonnet, similar to the Shakespearean style, starts with three quatrains and ends with a couplet, but the rhyme scheme differs from Shakespearean sonnets. The rhyme scheme interlocks with the quatrains -- ABAB BCBC CDCD EE. The turn or commentary about the subjects of the quatrains occurs in the couplet. An envelope sonnet follows the rhyme scheme ABBA CDDC and ends with EFGEFG or EFEFEF. It is similar to the Italian sonnet but uses different rhymes for two quatrains rather than one octave. Still other poets abandon the rhyme scheme of these traditional sonnets, such as Frederick Goddard Tuckerman’s “Sonnet XXVII,” which rhymes ABBABCABADECED. Similar to an Italian sonnet, the turn occurs around line 10. Other sonnets like the curtal or the stretched sonnet shorten or lengthen the form.