This paper treats the problems of total motion knee goniometry:​ goniometer design, attachment, data reduction and operating techniques to ensure accurate results. A new knee goniometer of the direct-attachment electromechanical type is described, and a computional technique for quantifying the total relative motion of the lower leg relative to the upper leg is developed. This entails six motion variables which must be reducible to three relative angular and three relative displacement variables, providing a more complete and precise quantification of knee motion than previously possible. Details of the spatial linkage and attachments are presented, along with operating experience demonstrating the importance of attachment technique. The goniometer is now in use as both a research and diagnostic tool. Typical results for normal and abnormal knees are presented to demonstrate the device and technique described.