Special isoparametric curves:

There are four special isoparametric curves such as  : p (0, v), p (1, v), p (u, 0) and p (u, 1). These are the boundary curves as they map the boundary of the unit square (that means the domain of the surface) to the surface. Setting up u to 0 and 1 and setting v to 0 and 1 yield the following equations of the boundary curves:

Thus, the boundary curve corresponding to u = 0 and u = 1 are Bézier curves described by the 0-th row and the m-th row of the given control points. Likewise, the boundary curve corresponding to v = 0 and v = 1 are Bézier curves described by the 0-th column & the n-th column of the given control points.