Abstract: Apparatus to determine the mutual positions of a femur head and a pelvis in the course of hip surgery in order to be able to make correct securement of prosthetic hip components. Elongated first and second orientation rods are releasably securable respectively to a femur head and a hip pelvis. A gauge rod (13) extends between the two orientation rods and is displaceable along one of the orientation rods. The gauge rod is releasably securable in any of a plurality of adjusted positions longitudinally of that one orientation rod, in order to control the mutual parallelism and the distance between the two orientation rods. The orientation rod other than that one orientation rod has gauge markings thereon for determining the position of the gauge rod along that other orientation rod.