The curve through points A, B, C, and D is defined by a parametric function [code]a(t)[/code].[br]The curve through points E, F, G, and H is defined by a parametric function [code]b(t)[/code].[br]The idea is to "stretch" the surface from one curve to the other, which is the purpose of the [code]u[/code] parameter.[br]The [code]u[/code] parameter varies from 0 to 1 (or in this app, 0 to [code]c [/code]which varies from 0 to 1). When [code]u = 1[/code], the surface is all [code]a(t)[/code]. When [code]u = 0[/code], the surface is all [code]b(t)[/code]. When [code]u = 0.4[/code], the surface stretches 40% of the way from [code]b(t)[/code] to [code]a(t)[/code].[br]The parameter [code]t[/code] varies from 0 to 1 for splines. This can vary as freely as you like. [br][br]This is the command:[br][code]Surface(u x(a(t)) + (1 - u) x(b(t)), u y(a(t)) + (1 - u) y(b(t)), u z(a(t)) + (1 - u) z(b(t)), t, 0, e, u, 0, c)[/code]