Radiologic assessment of fracture reduction in displaced supracondylar fractures of the elbow in children is notoriously difficult. The Baumann angle is often used as a guide to the adequacy of reduction. This is based on the assumption that the Baumann angle has a constant relationship to the carrying angle in a displaced fracture. The effect of rotation on the relationship between these angles has not been studied in detail. Computed tomography (CT) studies have shown that ≤40° of rotation may still be present after closed manipulation of a supracondylar fracture. A computer programme analysing vectors was designed to study the relationship between the Baumann angle and the carrying angle with predetermined displacements of the distal fragment. The relationship between the Baumann angle and the carrying angle was thus defined and, in contrast to the literature, found to be more complex than previously appreciated. The Baumann angle was found to be an inaccurate indicator of the carrying angle when treating displaced supracondylar fractures.