The window "Domain" displays a domain as a wireframe, a point (u0, v0) in the domain, and the u-constant and v-constant curves through that point in red and green, respectively.

The window "Surface X(u,v)" shows the torus of revolution X(u,v) for the given domain, the images of the point and the curves from the "Domain" window, and the tangent vectors Xu(u0,v0) and Xv(u0,v0) to the surface at the point X(u0,v0).

Checking the "Show tangent plane" checkbox will show the tangent plane to X(u,v) at the point X(u0,v0). It is possible to change u0 and v0 in the control window. It is also possible to enter any of the functions listed in the control window for X(u,v) and to change the parameters used to define these functions.