1. A computer system for permitting interaction between a user of the computer system and a three-dimensional computer model, comprising: a processor for storing the model, a mapping defining a geometrical correspondence between the model and a real world workspace, and data defining a three-dimensional editing volume of the workspace; display means for rendering a 3D image of that portion of the model within the editing volume; at least one first input device for operation by a first hand of a user to input first signals to the processor, the processor being arranged to do any of: modify the geometrical correspondence between the model and the workspace, modify the orientation of the editing volume relative to the workspace and translate or rotate said portion of the model relative to the editing volume, in response to said first signals; and at least one second input device for operation by a second hand of the user to input second signals to the processor, the processor being arranged to do any of: modify the 3D position of the editing volume relative to the workspace and modify or operate upon a portion of the model, in response to said second signals, wherein the boundaries of the editing volume are at least one of: continuously displayed, displayed when changed, displayed upon any translation or rotation of the editing volume or selection of an active face, displayed upon change of the geometrical correspondence between the model and the workspace, displayed upon input of any signals to the processor via the first input device, and displayed upon user command, and wherein the first signals can only be input via the first input device and the second signals can only be input by the second device such that at any given time each hand of the user controls a specific and distinct set of interactive functionalities.