The greater the stimulus generalisation, the less stimulus control is said to be exerted by a discriminative stimulus. Many textbooks of psychology suggest that stimulus generalisation gradients always take the form of a symmetrical curve, with the amount of responding decreasing as a function of increasing distance from the training stimulus. Gradients of generalisation have been investigated using many different stimulus dimensions. H. S. Terrace's findings are exceptionally important, for they challenge any theory of discrimination learning which assumes that errors are crucial in the acquisition of differential control by stimuli or in sharpening the stimulus control exerted by a discriminative stimulus. The combination of schedules of intermittent reinforcement and techniques for developing stimulus control provides a most powerful technique for the study and control of operant behaviour. The patterns of behaviour controlled by multiple schedules of reinforcement show that the animals are capable of distinguishing between the discriminative stimuli associated with the different component schedules.