Four, three-dimensional, photoelastic, frozen-stress models of keyed connections were loaded in torsion. This technique was applied to study the axial shear stress distributions in both solid and hollow, side milled and end-milled keyed connections. The dimensions of the key cross-section were according to the British Inch Standard. The coefficient of friction between the contact surfaces of shaft and hub was measured at the stress-freezing temperature. The surface conditions were arranged to simulate the typical prototype friction. Positions and magnitudes of the peak shear stresses were determined at the different positions of contact between key and shaft keyway. Results from all models are compared and a new key design is described.