According to Winthrop University, a rhyme scheme in poetry or song is the arrangement of rhymes by line. Rhyme, as a sound device, is determined purely by pronunciation and not by spelling.
Types of rhyme schemes are determined by which lines rhyme and by the total line count. A common type of rhyme scheme is the sonnet. Sonnets are usually fourteen lines, which creates a challenge to the author when deciding upon rhyme, given the limited space. The English sonnet, made famous by Shakespeare, is derived from the Petrarchan sonnet, an Italian form. The Petrarchan sonnet's lines are divided into the octet (first eight lines) and the sestet (final six lines).