Directrix (Ruled Surface)

The curve ${\bf b}(u)$ in the Ruled Surface parameterization

$${\bf x}(u,v)={\bf b}(u)+v{\bf d}(u)$$

is called the directrix (or Base Curve).

See also Director Curve, Ruled Surface

References

Gray, A. Modern Differential Geometry of Curves and Surfaces. Boca Raton, FL: CRC Press, p. 333, 1993.