There are different levels of continuity. A curve with an angle or cusp in it is C0 continuous -- that is, the curve is continuous but has no derivative at the cusp. A curve with no such cusps but whose curvature changes is C1 continuous. Its derivative is also continuous, but its second derivative is not. A curve with uninterrupted, unchanging curvature is C2 continuous. Both its first and second derivatives are also continuous. A curve can have still higher levels of continuity, but for computer modeling these three are adequate. Usually the eye can't distinguish between a C2 continuous curve and one with higher continuity. Continuity and degree are related. A degree 3 equation can generate a C2 continuous curve. This is why higher-degree curves aren't generally needed in NURBS modeling. Higher-degree curves are also less stable numerically, so using them isn't recommended. Different segments of a NURBS curve can have different levels of continuity. In particular, by placing CVs at the same location or very close together, you reduce the continuity level. Two coincident CVs sharpen the curvature. Three coincident CVs create an angular cusp in the curve. This property of NURBS curves is known as multiplicity. In effect, the additional one or two CVs combine their influence in that vicinity of the curve. By moving one of the CVs away from the other, you increase the curve's continuity level again. In 3ds Max, multiplicity also applies when you fuse CVs. Fused CVs create a sharper curvature or a cusp in the curve. Again, the effect goes away if you unfuse the CVs and move one away from the other.