The principal axes can be completely defined relative to the fixed set by three angles: the two angles \((\theta, \phi)\) fix the direction of \(x_{3}, \text { but that leaves the pair } x_{1}, x_{2}\...The principal axes can be completely defined relative to the fixed set by three angles: the two angles \((\theta, \phi)\) fix the direction of \(x_{3}, \text { but that leaves the pair } x_{1}, x_{2}\) free to turn in the plane perpendicular to \(x_{3}\), the angle \(\psi\) fixes their orientation.
