These letters indicate exactly where the rhymes take place at the conclusion of the traces. In ABAB, the primary and third lines rhyme at the end, as do the second and fourth traces. In ABCB, just the next and fourth traces rhyme at the tip. The dictionary definition of Rhyme: https://typesofsticker28405.jiliblog.com/84772742/top-latest-five-types-of-rhymes-in-poems-urban-news