What is isogonal family of a given family of curves? I searched in Wikipedia  isogonal trajectories  about the definition but I do not understand what does it mean by fixed "angle". Angle with the tangents of the curves? Clockwise angle? Orientated Angle? Thanks in advance. 

I think that for two curves $y=f(x)$ and $y=g(x)$ which intersect at $(x_0,y_0)$, they are defining the angle between the curves to be $\ \mathrm{arctan}(f'(x_0)) - \mathrm{arctan}(g'(x_0))$.
