Since the advent of the use of roentgen rays in the diagnosis of cardiac disturbance, accurate measurement of the size of the heart has become a routine procedure. In the attempt to distinguish between the normal and the abnormal cardiac silhouette various measurements have been suggested,1 of which the transverse diameter, because of the ease and accuracy of determination, has received the widest favor. However, it was found that average or arbitrarily set limits of the normal width were not reliable as a guide to cardiac enlargement in the individual instance. In 1919, Danzer2 proposed the ratio of the transverse diameter of the heart to the internal diameter of the chest, determined on a roentgen-ray shadow on a film taken 6 feet (1.8 meters) from the tube, as a criterion of the normal size of the heart. He believed that there is a constancy of relationship between the