This paper is a diachronic study of post-adjectival infinitive constructions with adjectives denoting goodness, fitness, or necessity. Traditionally these constructions are analysed astough-constructions. They can be divided into two semantically and syntactically distinct types, viz. activity-oriented and characteristic-oriented constructions. This distinction can be used to explain the variation in clausal voice type that is found with the post-adjectival constructions from the Late Middle English period onwards. In addition, a distinction needs to be made between adjectives expressing necessity, which only occur in activity-oriented constructions, and those expressing goodness or fitness, which may occur in the two construction types. Voice variation is only meaningful for the latter type of adjectives: in characteristic-oriented constructions they tend to be construed with an active infinitive, whereas in activity-oriented constructions they appear with a passive infinitive. As such, the distinction between these construction types helps to explain formal properties of the post-adjectival infinitive construction.