The window "Domain" displays a domain as a
wireframe, a point (u_{0}, v_{0}) 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 surface X(u,v) for
the
given domain, the images of the point and the curves from the
"Domain" window, the tangent vectors X_{u}(u_{0},v_{0})
and X_{v}(u_{0},v_{0}) to the
surface at the point X(u_{0},v_{0}), and the unit
normal
vector

N(u_{0},v_{0}) = (X_{u}(u_{0},v_{0})
× X_{v}(u_{0},v_{0}))/|X_{u}(u_{0},v_{0})
× X_{v}(u_{0},v_{0})|
to the
surface at the point X(u_{0},v_{0}).

Checking the "Show
tangent plane" checkbox will show the tangent plane to X(u,v) at the
point
X(u_{0},v_{0}). (Note that this box is checked by
default.) It is possible to change u_{0}
and v_{0} 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.