The difference between plane metal and curved metal is different in definition and expression. Plane means that the whole connecting line of any two points on the plane falls on this plane, which is a two-dimensional zero curvature continuation. Such a plane is a straight line that intersects with similar planes at will, and a curved surface can be regarded as a trajectory formed by the continuous movement of a moving straight line (straight line or curve) in space. The moving line that forms a surface is called a generatrix, which is expressed in different ways: the plane is usually drawn as a parallelogram, which extends indefinitely, and the parallelogram only represents a part of the plane. This is the same as just drawing a paragraph to represent a straight line. Sometimes a plane can be represented by a triangle or a closed graph as needed. The representation of a surface is similar to that of a plane, and the most basic requirement is to project and determine the geometric elements of the surface.