Cone (geometry).

Cone (geometry). Cone (geometry), in geometry, surface generated by a straight line that moves along a closed curve while always passing through a fixed point. The straight line is called the generatrix, the fixed point is called the vertex, and the closed curve is called the directrix. If the generatrix is of infinite length, it generates two conical surfaces on opposite sides of the vertex. If the directrix of the cone is a circle, the cone is usually referred to as a circular cone. Mathematicians study oblique cones (in which the axis is not perpendicular to the plane of the base) and various noncircular cones (in which the base is in the form of an ellipse or some other closed curve). The surface that is commonly called a cone, however, is the right circular cone. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.