High Quality Content by WIKIPEDIA articles! In mathematics, 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 to C at these points. That is, L is an tangent line at P and at Q. In general, an algebraic curve will have infinitely many secant lines, but only finitely many bitangents. Bezout's theorem implies that a plane curve with a bitangent must have degree at least 4. The case of the 28 bitangents of a quartic was a celebrated piece of geometry of the nineteenth century, a relationship being shown to the 27 lines on the cubic surface.

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