Dihedral angles - how to calculate

[George A. Heavner]

I am working on a project where I have cartesian coordinates for four
consecutive atoms and need to calculate the dihedral angle (with correct
sign).  I am not a crystallographer and readily admit to difficulty with
the math.  The magnitude is not difficult (angle between two planes) but
I am stuck on the sign.  Can anyone provide me with the procedure or
point me to a readily intelligible text or other source.

Thanks

    George A. Heavner