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 Möbius strip X(u,v) = ((2 – vcos(u/2))cos(u), (2 – vcos(u/2))sin(u), vsin(u/2)) for the given domain, the images of the point and the curves from the "Domain" window, the tangent vectors Xu(u0,v0) and Xv(u0,v0) to the surface at the point X(u0,v0), and the unit normal vector
N(u0,v0) = (Xu(u0,v0) × Xv(u0,v0))/|Xu(u0,v0) × 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). (Note that this box is checked by default.) It is possible to change u0 and v0 in the control window.