The Italian sonnet is sometimes also known as the Petrarchan sonnet after the 14th-century Italian poet Francesco Petrarca, or Petrarch. The rhyme scheme for the first eight lines of the Italian sonnet is abbaabba, so that the first, fourth, fifth, and eighth lines rhyme, as do the second, third, sixth and seventh lines. This set of eight lines is called the octave; the second set, of six lines, is called the sestet. The rhyme scheme for the sestet can be cdcdcd, cdecde, cdedce or slight variations on these arrangements, as long as the last two lines do not rhyme. Traditionally, the octave sets forth a problem, and the sestet resolves it.
The English sonnet is also sometimes called the Shakespearean sonnet after William Shakespeare. While still 14 lines long and written in iambic pentameter, the English sonnet consists of four sets of four lines, known as quatrains, followed by two lines, known as a couplet. The rhyme scheme is abab cdcd efef gg. The final couplet sums up or offers a commentary on the subject of the sonnet.
Among English sonnets, a variation on the Shakespearean sonnet is the Spenserian sonnet, named for 16th-century English poet Edmund Spenser, who linked the quatrains in his sonnets by using the rhyme scheme abab bcbc cdcd ee. Spenser's sonnets were less concerned with the structure in which a problem is set up and then resolved than the Italian or Shakespearean sonnets were; his quatrains more often present three separate but interrelated ideas, with the final couplet adding another idea or summing up the entire sonnet.
Most variations on the sonnet form, especially modern ones, play with the rhyme scheme, sometimes seeming to follow no pattern at all. Over the centuries, some poets have also used different meters from the standard iambic pentameter, particularly poets writing in languages other than English. Petrarch and Shakespeare both wrote series, or sequences, of sonnets on related topics --- generally, on love --- but some poets have written individual sonnets reflecting on whatever topic moved them at the moment.