A term used in linguistics to refer to the process or result of replacing one item by another at a particular place in a structure. In grammar, the structural context within which this replacement occurs is known as a substitution frame. The set of items which can be used paradigmatically at a given place is known as a substitution class. A word which refers back to a previously occurring element of structure may be called a substitute word. In government-binding theory, substitution is one of the two main types of movement process (the other being adjunction); it involves the moved category replacing an empty category of the same kind in accordance with the structure-preserving constraint. In language teaching, exercises to improve the ability of learners to carry out a process of item replacement are known as substitution drills (or ‘pattern drills’).