Bitangent

This article is about the geometric concept. For the basis vector often called the "bitangent" in computer graphics, see Frenet–Serret formulas.
The Trott curve (black) has 28 real bitangents (red). This image shows 7 of them; the others are symmetric with respect to 90° rotations through the origin and reflections through the two blue axes.

In geometry, a bitangent to a curve C is a line L that touches C in two distinct points P and Q and that has the same direction as C at these points. That is, L is a tangent line at P and at Q.