The window "Domain" displays a domain as a wireframe, a point (u(t0), v(t0)) in the domain, the u-constant and v-constant curves through that point in red and green, respectively, and the curve (u(t), v(t)) in blue.

The window "Surface X(u,v)" shows the surface X(u,v) for the given domain, the images of the point and the curves from the "Domain" window, the tangent vectors Xu(u(t0),v(t0)) and Xv(u(t0),v(t0)) to the surface at the point X(u(t0),v(t0)), and the tangent vector Xu(u(t0),v(t0))u'(t0) + Xv(u(t0),v(t0))v'(t0) to the curve X(u(t), v(t)) at t = t0.

Checking the "Show tangent plane" checkbox will show the tangent plane to X(u,v) at the point X(u(t0),v(t0)). Note that the tangent vector Xu(u(t0),v(t0))u'(t0) + Xv(u(t0),v(t0))v'(t0) to the curve X(u(t), v(t)) at t = t0 is contained in the tangent plane. It is possible to change u0, v0, and X(u,v) 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.