A rhyme scheme is a pattern of rhymes at the end of each line of a poem or song. The most common rhyme schemes are ABBA, ABABA, AABA, ABAB, and ABAC, which all consist of four or five letters. ABBA is a rhyme scheme in which the first and fourth lines rhyme, as do the second and third lines. ABABA is a rhyme scheme in which the first, third, and fifth lines rhyme, as do the second and fourth lines. AABA is a rhyme scheme in which the first, second, and fourth lines rhyme, while the third line does not. ABAB is a rhyme scheme in which the first and third lines rhyme, as do the second and fourth lines. Finally, ABAC is a rhyme scheme in which the first, third, and fourth lines rhyme, while the second line does not.