Based on the geometry characteristics of Bertrand surfaces, the complicated surface conjugation issue can be discussed with their directrix line. 基于白川德曲面的几何特征，将复杂的曲面共轭问题归结到准线上来讨论。
The surface generated by a straight line,the generator,passing through a fixed point,the vertex,and moving along a fixed curve,the directrix. 锥面由一条直线，即母线，通过一固定点，即顶点，沿一给定曲线，即准线，连续移动而产生的平面。
美
[dɪ'rektrɪks]
