The ratio of the length of two line segments on a line stays unchanged. As a special case, midpoints are mapped on midpoints. The length of a line segment parallel to the projection plane remains unchanged. The length of any line segment is shortened if the projection is an orthographic one. Any circle that lies in a plane parallel to the projection plane is mapped onto a circle with the same radius. Any other circle is mapped onto an ellipse or a line segment (if direction is parallel to the circle's plane). Angles in general are not preserved. But right angles with one line parallel to the projection plane remain unchanged. Any rectangle is mapped onto a parallelogram or a line segment (if is parallel to the rectangle's plane). Any figure in a plane that is parallel to the image plane is congruent to its image. Types Orthographic projection Orthographic projection is derived from the principles of descriptive geometry, and is a type of parallel projection where the projection rays are perpendicular to the projection plane. It is the projection type of choice for working drawings. The term orthographic is sometimes reserved specifically for depictions of objects where the principal axes or planes of the object are also parallel with the projection plane (or the paper on which the orthographic or parallel projection is drawn). However, the term primary view is also used. In multiview projections, up to six pictures of an object are produced, with each projection plane perpendicular to one of the coordinate axes. However, when the principal planes or axes of an object are not parallel with the projection plane, but are rather tilted to some degree to reveal multiple sides of the object, they are called auxiliary views or pictorials. Sometimes, the term axonometric projection is reserved solely for these views, and is juxtaposed with the term orthographic projection. But axonometric projection might be more accurately described as being synonymous with parallel projection, and orthographic projection a type of axonometric projection. The primary views include plans, elevations and sections; and the isometric, dimetric and trimetric projections could be considered auxiliary views. A typical (but non-obligatory) characteristic of multiview orthographic projections is that one axis of space usually is displayed as vertical. When the viewing direction is perpendicular to the surface of the depicted object, regardless of the object's orientation, it is referred to as a normal