An AABB rhyme scheme is a poem in which each section of four lines are divided into two couplets. Each of these couplets subsists of two rhyming lines, which creates a rhyme scheme that is present throughout the duration of the poem.
Rhyme schemes are present in various forms of poetry to provide tempo and meter within the piece. In addition, many poems rely on various types of rhyme schemes to provide a structure throughout each stanza.
AABB is a common rhyme scheme that is most prominently featured in four-line poems but can also be used within longer poems to establish a particular feel or rhythm. Within this rhyme scheme, each individual couplet is two lines that rhyme. Each line within the stanza either adheres to the rhyme of "A" lines or "B" lines, in accordance with the rules of the rhyme scheme.
The cat on a mat - A
Played with a hat - A
Underneath the sun - B
On a great day of fun - B
Despite being most commonly used in poetry, this type of rhyme scheme can be used in other literary works, such as song writing. Often, literary works contain more than one rhyme scheme to change the tempo in longer pieces.