The answer is: the rhyme scheme of a sonnet
The rhyme scheme is the patter used by the author to make the lines of the poem rhyme, for each ending pattern of a line is assign a letter, let analyze the following stanza from “ME, MYSELF, & I” as an example:
The coolness of the night A
Refreshes my skin. B
The stars shine so bright, A
Causing me to grin. B
After identifying the first ending “ight” and assign a letter to the sound “A”, you can proceed with the second, where the ending is “in” and assign the letter “B”, then in the third line the ending is the same that in the first one so the letter is repeated and the same with the second and forth line obtaining a ABAB rhyme scheme. This is how the letters of the alphabet are used to defined the rhyme scheme of a sonnet .
