Techniques are described for providing an environment to model and depict a three-dimensional (3D) representation of a patient's dental arch, i.e., a virtual dental arch, and a separate cross section tool, such as a graphical user interface (GUI), as a visual aid to an orthodontic practitioner for selecting a position of cross section planes relative to the virtual dental arch. The GUI may display a control image and two moveable parallel lines. The position of the parallel lines relative to the control image approximates the position of the cross section planes relative to the virtual dental arch. Thus, by interacting with the GUI, the practitioner is able to change the position of the cross section planes within the 3D environment. Consequently, the practitioner can visualize the cross sections of the virtual dental arch within the 3D environment while selecting the position of the cross section planes.