The author considers nonuniform rational \(B\)-spline (NURBS) curves which are rational one-parametric curves in 3D of order \(k\). The envelope of a family of NURBS curves of order \(k\) is a curve defined by the properties that at every one of its points it is tangent to at least one curve of the family and, secondly, it is tangent to every curve of the family at least at one point. The main result of the paper states that for \(k>3\), every segment of a NURBS curve of order \(k\) is the envelope of a family of NURBS curves of order \(k-s\) (\(s=1,2,\dots,k-2\)). Furthermore, this envelope is uniquely determined by the family of NURBS curves of order \(k-s\). Thus rational \(B\)-spline curves can directly be defined in a geometric way. As a simple corollary one obtains the analogous well-known results for ordinary \(B\)-spline curves.