[…] let us allow certain features to be designated in the form ['X'—'Y'], where 'X' and 'Y' are strings (perhaps null) of symbols. We shall henceforth call these 'contextual features'. - Chomsky (1969), a pag.93
[…] features introduced by strict subcategorization or selectional rules (what we have called the "contextual features"). - Chomsky (1969), a pag.111