Source: Wikipedia
Note: In research papers, ratings are usually converted to numerical values, from 1 to 22, except:
Alissa et al. (2013) [1683], who use 16 to 1, stopping at B-.
Cornaggia and Cornaggia (2013) [1283] use not only a 21 to 0 ranking (then converted into financial health rapid rating from 95 to 5), but also 9-point scale based on Moody's scale (where AAA = 9).
Gutierrez et al. (2020) [2166] use 1 to 10 (for D).
Devos (2014) [1651] use 24 to 1 (for D).
Shi, Wang and Zhang (2017) [1670] use 1 to 20 (for C), due to their sample characteristics.
Shaker (2014) [0275] use 2 to 24 (for C).
Zhang (2010) [0381] and Löffler and Maurer (2011) [1441] use 1 to 21 (for C).
Goyal and Wang (2013) [1503] use 20 to 1.
Elyasiani, Jia and Mao (2010) [1723] use 2 to 21 on the Moody's scale.