A pelvic locating frame, adapted to adjustably mount in opposition to an adjustable backrest, and suitable for use in connection with computer assisted surgery using a tracking system for finding the orientation of a human patient's anterior pelvic plane, said pelvic locating frame comprising a support assembly including: a first and a second vertical elongated member (214, 216) which are slidably adjustable with respect to each other; a first anterior superior iliac spine locating feature coupled to said first vertical elongated member via a first horizontal elongated member, and adapted to engage in close relation to the patient's anterior superior iliac spine; a second anterior superior iliac spine locating feature coupled to said second vertical elongated member via a second horizontal elongated member, and adapted to engage in close relation to the patient's anterior superior iliac spine; at least one pubic locating feature coupled to said first vertical elongated member via a third horizontal elongated member which is slidably mounted to said first vertical elongated member, and adapted to engage in close relation to the patient's pubic symphysis, said anterior superior iliac spine features (223a, 223b) and said pubic locating feature defining an anterior pelvic plane; and a group of index features (252a, 252b, 252c) formed to engage a pointer of a trackable probe of the tracking system and constrained to maintain a predetermined rotational relationship to said anterior pelvic plane defined by said anterior superior iliac spine features (223a, 223b) and said at least one pubic locating feature, wherein said support assembly is mounted to a horizontal adjustment rod and a vertical adjustment rod, said horizontal and vertical rods being distinct from said support assembly with said support assembly being configured for rotation about an axis of said horizontal rod and about an axis of said vertical rod. a first and a second vertical elongated member (214, 216) which are slidably adjustable with respect to each other; a first anterior superior iliac spine locating feature coupled to said first vertical elongated member via a first horizontal elongated member, and adapted to engage in close relation to the patient's anterior superior iliac spine; a second anterior superior iliac spine locating feature coupled to said second vertical elongated member via a second horizontal elongated member, and adapted to engage in close relation to the patient's anterior superior iliac spine; at least one pubic locating feature coupled to said first vertical elongated member via a third horizontal elongated member which is slidably mounted to said first vertical elongated member, and adapted to engage in close relation to the patient's pubic symphysis, said anterior superior iliac spine features (223a, 223b) and said pubic locating feature defining an anterior pelvic plane; and a group of index features (252a, 252b, 252c) formed to engage a pointer of a trackable probe of the tracking system and constrained to maintain a predetermined rotational relationship to said anterior pelvic plane defined by said anterior superior iliac spine features (223a, 223b) and said at least one pubic locating feature,