View source for Focal net of a congruence

A net that is cut out on the focal surface of a congruence of lines (a hyperbolic one) by enveloping surfaces of that congruence. A focal net of a congruence is a [[Conjugate net|conjugate net]]. One family of lines of it consists of the cusps of one family of enveloping surfaces of the congruence; the other family is formed by the lines where the enveloping surfaces of the other family are tangent to the focal surface. Every conjugate net on a two-dimensional surface is a focal net of the congruence of tangents to the lines of one of the families of that net. Every hyperbolic congruence of lines has two focal nets.

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  S.P. Finikov,   "Theorie der Kongruenzen" , Akademie Verlag  (1959)  (Translated from Russian)</TD></TR></table>