The invention relates to a method of calibrating an instrument to be used in establishing a reference plane for hip arthroplasty on a hip, said instrument having first, second and third legs, each leg of said instrument having a tip, the method comprising the steps of:selecting first and second points to be used in establishing said reference plane, said first point being in the region of the posterior inferior acetabulum and said second point being in the area of the anterior superior iliac spine；determining the distance between said first and second points； andsetting said instrument so that the distance between the tips of the first and second legs equals the determined distance between the first and second points, and the distance between the tips of the first and third legs is a prescribed ratio of the determined distance between the first and second points.
