A system for use in performing hip arthroplasty, comprising: a computer processor; a first electronic position sensor capable of reporting information about its orientation in 3 -dimensional space to the computer processor; a second electronic position sensor capable of reporting information about its orientation in 3 -dimensional space to the computer processor; a leg length measurement unit, comprising (i) a reflection pin configured to be attached to a human subject's femur; and (ii) an electronic distance sensor capable of being attached to the human subject's pelvis and capable of reporting, to the computer processor, information about its distance from the reflection pin; characterised in that the system further comprises: an axis guide, comprising a substantially linear rigid bar at least as long as a human subject's pelvis is wide, constructed and arranged to have a slot at each end of the bar and at least two apertures adjacent and perpendicular to at least one of the slots, wherein each of the apertures is capable of receiving a pin guide and each pin guide is capable of receiving a securing pin; and application software capable of (i) receiving information from the first electronic position sensor, the second electronic position sensor, and the electronic distance sensor; (ii) calculating and causing to be displayed angular relationships derived from the position sensor information; and (iii) calculating and causing to be displayed leg length information derived from the distance sensor information. a computer processor; a first electronic position sensor capable of reporting information about its orientation in 3 -dimensional space to the computer processor; a second electronic position sensor capable of reporting information about its orientation in 3 -dimensional space to the computer processor; a leg length measurement unit, comprising (i) a reflection pin configured to be attached to a human subject's femur; and (ii) an electronic distance sensor capable of being attached to the human subject's pelvis and capable of reporting, to the computer processor, information about its distance from the reflection pin; characterised in that the system further comprises: an axis guide, comprising a substantially linear rigid bar at least as long as a human subject's pelvis is wide, constructed and arranged to have a slot at each end of the bar and at least two apertures adjacent and perpendicular to at least one of the slots, wherein each of the apertures is capable of receiving a pin guide and each pin guide is capable of receiving a securing pin; and application software capable of (i) receiving information from the first electronic position sensor, the second electronic position sensor, and the electronic distance sensor; (ii) calculating and causing to be displayed angular relationships derived from the position sensor information; and (iii) calculating and causing to be displayed leg length information derived from the distance sensor information. an axis guide, comprising a substantially linear rigid bar at least as long as a human subject's pelvis is wide, constructed and arranged to have a slot at each end of the bar and at least two apertures adjacent and perpendicular to at least one of the slots, wherein each of the apertures is capable of receiving a pin guide and each pin guide is capable of receiving a securing pin; and application software capable of (i) receiving information from the first electronic position sensor, the second electronic position sensor, and the electronic distance sensor; (ii) calculating and causing to be displayed angular relationships derived from the position sensor information; and (iii) calculating and causing to be displayed leg length information derived from the distance sensor information.