A rhyme scheme is the pattern of rhymes at the end of each line in a poem. It is often described using letters to represent sounds, where lines that rhyme are given the same letter. For example, in the rhyme scheme AABB, the first two lines rhyme with each other, and the next two lines rhyme with each other. Rhyme schemes can vary widely, creating different effects in poetry. Common schemes include ABAB, where alternating lines rhyme, and ABBA, where the first and fourth lines rhyme, as do the second and third. These patterns help to enhance the musical quality of the poem.