System of marking rhyme patterns with letters: AABB (couplet), ABAB (alternating), ABBA (enclosed), ABCABC (triple), etc.
Assign each end rhyme a letter — first rhyme = A, second = B, etc. ABAB = alternating. AABB = couplets. Complex schemes have 6+ different rhymes.
The rhyme scheme is the 'skeleton' of sound structure. Different schemes create different feelings.
Shakespeare: ABAB CDCD EFEF GG (sonnet). Eminem: complex schemes with internal rhymes.