The principal axes can be completely defined relative to the fixed set by three angles: the two angles (θ,ϕ) 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 (θ,ϕ) fix the direction of x3, but that leaves the pair x1,x2 free to turn in the plane perpendicular to x3, the angle ψ fixes their orientation.