We label citations by the last name of first author,
the year of publication, and an additional lower-case
letter. For example:
Hong1997a, Miller2001c, and Planck1901aThe additional lower-case letter is arbitrary in the sense that it is sequential in the order in which the paper was added to the bibliography. This is necessary to maintain label stability. Suppose Hong1997a was published in July in 1997. If sometime in the future we unearth an obscure paper published by Hong in January of 1997, it will be added to the bibliography with the label Hong1997b.