Lines

Surface intersected in lines are bounded by lines. Lines are either straight or curved. Examine the model of a rectangular prism. When two surfaces intersect, they do so in a straight line, called an edge of the prism. The curved surface of a cylinder and either of the plane surfaces intersect in a curved line. The trace made on paper by a fine pencil point may represent a line, but even the finest trace will be a geometrical solid, since it has length, breadth, and thickness, and a line has length, but no breadth and no thickness.