(ii) **Developable** surfaces, such as the cylinder and the cone, form a special class, so far as the calculation of their area is concerned.

The curved surfaces of the cylinder and of the cone are **developable** surfaces; i.e.

Such a curve may be considered as described by a point, moving in a line which at the same time rotates about the point in a plane which at the same time rotates about the line; the point is a point, the line a tangent, and the plane an osculating plane, of the curve; moreover the line is a generating line, and the plane a tangent plane, of a **developable** surface or torse, having the curve for its edge of regression.