The precise orientation of local coordinates is determined as follows:

• The vector pointing from "A" to "B" is defined to be parallel to the local X-axis.
• The cross product of vectors "AB" and "AC" defines a vector parallel to the local Z-axis, i.e., z = AB x AC.
• The cross product of vectors z and x defines a vector parallel to the local Y-axis, i.e., y = z x x.
• The origin of the axes is at the center (average) of the 4 joint locations (3 joint locations for a triangle).

The sign convention of output force and moment resultants is illustrated in Section 2.6.