Answer:
lowercase letters, such as ''abab cdcd''
Step-by-step explanation:
For example, the rhyme scheme for the English or Shakespearean sonnet form is abab cdcd efef gg.
the rhyme scheme for the Italian sonnet form popularized by Petrarch consists of an octave with the rhyme scheme abbaabba and of a sestet with one of several rhyme schemes, as cdecde or cdcdcd.
Note that some sources prefer to use uppercase letters though: ABAB.