abab
When determining rhyme scheme the first line always receives an A. If the next line rhymes with the first it earns a B. The third line receives a new letter if it doesn't rhyme with the previous lines. If it does have the same sound, then it receives the same letter as the line that matches, or rhymes, with it. Declines rhymes with shines so they both receive the label A. Dimm'd does not rhyme with shines so it receives a B. Untrimm'd rhymes with dimm'd so it receives a B. The rhyme scheme is abab.