“…Constraints examined in the literature include rigid motion [1,4,7,8,12,14,17], rigid fixedaxis motion [10,19], nonrigid fixed-axis motion [2], planar rigid motion [10], rigid motion about a vertical axis [15], certain bending motions [12], and rigid motion that conserves angular momentum [3]. This latter constraint led to the following theorem: Given three distinct orthographic projections of three points, (a) the projections are almost surely incompatible with any three-dimensional interpretation in which the points move rigidly and conserve angular momentum, but (b) if the projections are compatible with such an interpretation then, generically, they are compatible with at most two such interpretations.…”