A prescription for an orthodontic brace is selected by providing data representing a number of teeth of the dental arch and by displaying images of the teeth in two or more different positions. Images of at least one tooth when in the first position and when in the second position are simultaneously displayed in superimposed fashion, and any difference in orientations of the displayed tooth images between the first position and the second position appears in contrast. As a consequence, the relative difference in positions can be readily observed. The invention is particularly useful for selecting a prescription of a brace from two or more prescriptions, and is also useful for observing the relative effect of a single prescription when used on certain teeth in comparison to the position of the same teeth during an earlier stage of treatment.