The sonnet began as a "little song" with no defined structure (indeed, the free form sonnet's only prerequisite is that it be composed of fourteen lines). Today the sonnet is a poem that consists of fourteen lines and is often written in iambic pentameter (although some poets have been known to write sonnets in iambic hexameter). There exists many variations of the sonnet, but all of the following vary only in rhyme scheme (if there is one):

Primarily used for the purposes of love, the sonnet originated in Italy where Petrarch, the first person known to use the sonnet in structured form and after whom the Petrarchan Sonnet is named, lived. Edmund Spenser, the "Poet's Poet", adapted the Petrarchan Sonnet, giving it a new rhyme scheme and bringing the sonnet into English. The new sonnet was dubbed the Spenserian Sonnet, after its creator. Further revisions were made by William Shakespheare, changing the rhyme scheme again to make the sonnet easier to write.