A regular surface can be defined as the path of a moving line. If the line is straight it is known as a ruled surface. The line that moves to generate a surface is the generatrix and in any one the generatrix is known as an element. The movement of the generatrix, to generate a surface, may be in any direction except in a straight line, like an arrow.
The simplest ruled surface is a plane that can be described several ways. For example, two parallel lines form a plane, as do two intersecting lines. A line segment rotating about one of its ends forms a plane. Any enclosed area forms a plane, such as triangles or any polygon by definition. To be concise, a plane is present when two elements are parallel or intersect.
A solid may either be formed by combining planes or generated from a single- curved line. A single-curved surface occurs when the generatrix, a straight line, is guided or directed by a single-curved line, called the directrix. A single-curved surface, such as the cone or the cylinder, can be developed. A cylinder may have a variety of shapes dependent upon the shape and position of the curved-line directrix. When the directrix is a circle and the elements revolve parallel to the center axis, the cylinder is known as a right cylinder.