A mathematical treatment of the errors in the crystal and counter settings, calculated by the mechanical analogue of the Bragg equation in point-drive (or linear) diffractometers, is presented. The errors are classified in two groups: those which are common to all point-drive types, and those which are peculiar to particular models. Displaying them in terms of the counter position enables the two categories of error to be simply combined. The application of the method to the Arndt and Phillips, and the Potter, diffractometers is shown. The development is discussed of an improved version of the Potter mechanism, the performance of which offers a real alternative to existing types.