Abstract: A device for determining the mean cardiac axis for aiding in the interpretation of an electrocardiograph includes a disc shaped member having a pair of radially extending arms pivotally attached to the center of the disc. The disc also carries a plurality of radial line markings thereon and preferably spaced every fifteen degrees. At least two of the radial markings, the one at 0.degree. and the one at 120.degree. representing the first and third EKG leads, have millimeter graduations thereon. Each arm has an elongated slide thereon which when positioned at the proper magnitude setting along the proper radial marking intersects the other slide to indicate the cardiac axis.