Hi Brian,
> So the xaxis and yaxis direction vectors used to create the
> new frame would be (or could be) in world coordinate frame.
Yup, normally those direction vectors are in world coordinates. The origin is also in world coordinates  the whole package of data gives the position and orientation of the frame's coordinate system.
> A new yaxis, parallel to the world coordinate yaxis would
> have the same vector direction, (0,1,0).
Yup that's correct.
> The new xaxis vector direction can be calculated from
> the two points, mathematically, or with vectorMath...
Yup, but the xaxis vector should be one that is at a 90 degree angle to the y axis vector. But it can be pivoted around the xaxis, like for example here in red are different possible xaxis lines that would work with the given green y axis:
 Michael
