To modify the shape of a surface, one can change the position of control
points, and remove a row or a column of control points. If the surface
is a rational Bézier or a NURBS surface, one can change the weight
of a control point. If the surface is a B-spline or a NURBS, one can also
change the positions of knots. Note that one cannot add control points;
but, one can add knots by knot insertion ** without** changing
the shape of the surface.

B-spline and NURBS surfaces involve knots. Information about knots are
shown in the ** Tracing Window**. In the following figure,
the horizontal direction is the

Suppose a surface is denoted as **S**(*u*,*v*). Then, fixing
a knot value *u _{i}* in the

Changing the value of knot will change the shape of the surface.
Dragging the little triangle of a knot will change its value. As a little
triangle moves, its corresponding knot value changes accordingly.
The following figures show the result of moving the knots 0.25 and 0.75
in the *u*-direction to 0.4 and 0.6, respectively, and knots 0.3, 0.5,
0.6 and 0.8 in the *v*-direction to 0.1, 0.2, 0.3 and 0.4, respectively.
The change on the surface is not very clear; but, you can see some part
of the surface becoming flatter (*e.g.*, the part marked by the
white rectangle).

If a knot is moved on top of another, a multiple knot is created.
Continue with the above example, if the knot 0.4 in the *u*-direction
is moved to 0.5, making 0.5 a knot of multiplicity 2, and the knots 0.2 and
0.3 in the *v*-direction are moved to 0.1 and 0.5, respectively,
making them multiple knots of multiplicity 2, we will have the following
figure. You will see straight edges and the surface seems locally flat.

As a rule of thumb, changing knot values does not always yield satisfactory surface shape. In fact, it is hard to predict the change of surface shape as the result of changing the values of knots.