One embodiment of a system described herein can include an axis guide for placement over a patient's right and left ASIS and at least two guides attachable to the axis guide to provide a through hole that is perpendicular to the axis guide. The system can further include at least two probes that can fit through the through holes and extend beyond an end of the axis guide by a predetermined distance, as well as at least two 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 patient's pelvis and the probes can be replaced by the pins to mount the pins to the patient's pelvis in a known orientation.