Tangents and Normals

Tangents are lines just touching a given curve and its Normal is a line perpendicular to it at the point of contact (or point of Tangency).

Taking the common case of a Circle, the Normal to a Tangent from a point P on the circumference is a line joining the point to the circle centre - and the Tangent is at right angles to the Normal. This formas the basis for methods of constructing Tangents such that the Point of Tangency is given accurately.