In many languages there are also bound substitutes, usually inflectional affixes. [...]. The variety of linkage by inflection called cross-reference regularly involves bound substitutes. Even the vestigial inflection of English verbs for person and number of subject –involving, usually, only the ‘-s’ suffix for the third person singular subjects in the present tense– conforms to this: the ‘-s’ is a bound substitute. (259-260)
- Hockett (1958), Pag. 259