A referencing apparatus for application onto a patient at least partially covered by surgical drapes, the referencing apparatus comprising: a 'T' shaped frame comprising three elongate members respectively attachable adjacent three extremities of the frame; a plurality of locating elements , each locating element attached to a distal end of respective elongate members , and shaped for engagement with the surgical drapes such that the surgical drapes are disposed intermediate the plurality of locating elements and a respective plurality of predefined anatomical sites on the pelvis of the patient, each of said locating elements being a circular loop having an interior defining a cavity; a proximal end of each elongate member is attachable to the frame , and a docking station coupled to an upper surface of said frame , and having an upstanding side perpendicular to the upper surface of the frame and including first and second holes extending therein at different orientations relative to each other, and, whereby, in use, the apparatus is shaped such that coupling of the docking station to the upper surface of the frame causes the docking station to assume a reference orientation relative to the plurality of predefined anatomical sites, wherein at least one of the cavities of the locating-elements is sized to receive a projection of the surgical drapes caused by the respective anatomical site; an electronic orientation monitor having a shaft ; characterized in that the first hole has an inclination arranged to match a desired inclination for an acetabular cup that is for insertion into the patient's left hand side acetabulum and the second hole has an inclination arranged to match a desired inclination for an acetabular cup that is for insertion in the patient's right hand side acetabulum, and wherein the first and second holes are sized and shaped to receive the shaft of the electronic orientation monitor . a 'T' shaped frame comprising three elongate members respectively attachable adjacent three extremities of the frame; a plurality of locating elements , each locating element attached to a distal end of respective elongate members , and shaped for engagement with the surgical drapes such that the surgical drapes are disposed intermediate the plurality of locating elements and a respective plurality of predefined anatomical sites on the pelvis of the patient, each of said locating elements being a circular loop having an interior defining a cavity; a proximal end of each elongate member is attachable to the frame , and a docking station coupled to an upper surface of said frame , and having an upstanding side perpendicular to the upper surface of the frame and including first and second holes extending therein at different orientations relative to each other, and, whereby, in use, the apparatus is shaped such that coupling of the docking station to the upper surface of the frame causes the docking station to assume a reference orientation relative to the plurality of predefined anatomical sites, wherein at least one of the cavities of the locating-elements is sized to receive a projection of the surgical drapes caused by the respective anatomical site; an electronic orientation monitor having a shaft ; characterized in that the first hole has an inclination arranged to match a desired inclination for an acetabular cup that is for insertion into the patient's left hand side acetabulum and the second hole has an inclination arranged to match a desired inclination for an acetabular cup that is for insertion in the patient's right hand side acetabulum, and wherein the first and second holes are sized and shaped to receive the shaft of the electronic orientation monitor . <EOS>