Abstract: This paper describes a comprehensive experimental evaluation of a two-handed free-form surface editor called THRED, which uses a pair of Polhemus 3D trackers with added buttons in a complementary two-handed style. On top of the underlying free-form surface editor application was built two other user interfaces that provide reasonable competition for the two-handed style. The second interface uses one button-enhanced 3D tracker in the dominant hand, with the non-dominant hand selecting commands from the keyboard. The third style is a mouse-based interface that is a simplified clone of the Alias modeler. This user study evaluates these interfaces in terms of pain and fatigue. The results show that experienced minimal pain and fatigue with THRED, an a par with that experienced in the mouse-based interface, but there was statistically significant fatigue in the use of the One-Handed interface. The pain and fatigue surveys clearly indicate that THRED and the Mouse-Based interface yield low discomfort, which contradicts the established wisdom that bat-based interfaces are likely to be painful or fatiguing to use.