Dear all, How could one find a tangential derivative of a function z=f(x,y) at a give point (x0,y0)? Is it always zero? I tried to go through in the following way. Well, if I need a tangential derivative f_t of a function f(x,y) I have to use a directional derivative in tangent direction, i.e. f_t=grad(f) . t, where t is the tangential vector. How one could find tangential vector t at a given point? I think, one has to find a tangential plane at a given point. The tangential plane at a given point (x0,y0) is fx(x-x0)+fy(y-y0)+f(x0,y0)=0. All the tangential vectors live on the tangential plane, i.e. they are perpendicular to the gradient of f(x,y) what means that f_t=grad(f) . t=0. Do I have a gap in my understanding. Thanks a lot in advance!