The principles of geometrical projection can be demonstrated with a drawing aid which preferably can be assembled from a kit of parts and which comprises an elongate base (10) carrying a number of object and image elements which can be set at different positions along the base and in different attitudes. These elements may comprise for example a sighting rod (19), a transparent sheet-form element (17) which defines a picture plane and on which the user draws images, an object (15), and a reflector (18). Perspective, orthographic and oblique projections can be demonstrated with just the one instrument using appropriate slots and holes to mount the elements.