These letters indicate exactly where the rhymes take place at the end of the lines. In ABAB, the initial and third lines rhyme at the end, as do the second and fourth lines. In ABCB, just the 2nd and fourth strains rhyme at the top. You are very likely familiar https://devinikhbs.blogaritma.com/27023071/the-single-best-strategy-to-use-for-types-of-rhymes-in-poems