Tangency

As it passes through the point where the tangent line and the curve meet, called the point of tangency, the tangent line is "going in the same direction" as the curve, and is thus the best straight-line approximation to the curve at that point.

If we reciprocate such a polyhedron about its intersphere, the dual polyhedron will share the same edge-tangency points and so must also be canonical; it is the canonical dual, and the two together form a canonical dual pair.

In the field of computer graphics, these three levels are sometimes called G 0 (continuity of position), G 1 (continuity of tangency), and G 2 (continuity of curvature).

Normal line to a curve The line perpendicular to the tangent line to a curve at the point of tangency is called the normal line to the curve at that point.

The extouch triangle of a reference triangle has its vertices at the points of tangency of the reference triangle's excircles with its sides (not extended).