A system includes an axis guide for placement over a patients right and left anterior superior iliac spine (ASIS) and guides attachable to the axis guide to provide a through hole that is perpendicular to the axis guide. The system can include probes that can fit through the through holes and extend beyond an end of the axis guide by, and bone-engaging pins that fit through the through holes. When the guides are assembled to the axis guide and the probes are assembled to the guides, the axis guide can be placed so that the probes contact the patients pelvis and the probes can be replaced by the pins to mount the pins to the patients pelvis in a known orientation.
