The window "Domain" displays a domain as a
wireframe, a point (u0, v0) in the domain, and
the u-constant and
through that point in red and green, respectively.
The window "Surface X(u,v)" shows the surface X(u,v) for
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
N(u0,v0) = (Xu(u0,v0)
surface at the point X(u0,v0).
Checking the "Show
tangent plane" checkbox will show the tangent plane to X(u,v) at the
X(u0,v0). (Note that this box is checked by
default.) It is possible to change u0
and v0 in the control window. It is also possible to enter
the functions listed in the control window for X(u,v) and to change
the parameters used to define these functions.