10. Marking a Poem’s Rhyme Scheme Lowercase letters are used to indicate the rhyme scheme in a poem. The a rhyme is –un in sun and dun , and the b rhyme is –ed in red and head . a b a b My mistress’ eyes are nothing like the sun; Coral is far more red than her lips’ red; If snow be white, why then her breasts are dun; If hairs be wires, black wires grow on her head.