[…] we have the following general principle holding of empty categories, that is, categories lacking phonetic content: (4) If α is an empty category, then (i) α is PRO if and only if it is ungoverned; (ii) α is trace if and only if it is governed.
- Chomsky (1993), a pag.136

[…] it was observed that the three types of empty category - PRO, variable, NP-trace - partition the class of possible positions of NP [...] If the three empty categories partition a certain distribution - in this case, virtually the distribution of NP - then it is reasonable to presume that in fact there is only one basic empty category α; each occurrence of α has one of three clusters of properties.
- Chomsky (1993), a pag.321