Guidelines for reference line positioning recommended for the training and evaluation software. In the radius, the reference line intersects (a) the peak when visible or (b) the mid-articular surface when the peak is not visible. Similarly, in the tibia, the reference line intersects (c) the peak when visible or (d) the flat plafond when the peak is not visible. In the radius, the hiker analogy easily explains the positioning of the reference line. The articular surface of the radius (e) can be considered as a hiking path (f). The hiker walks along the path (g) until he reaches the peak where he plants his flag, which coincides with the location where the reference line intersects the edge that represents the articular surface of the radius (h).