Surfaces and edges that are at an angle with all three planes of projection are called oblique. You will notice that an oblique surface will never be shown as a line or as a true size surface on any of the principal views. To show an oblique surface true size, you would first have to draw an auxiliary view to show the edge view of the surface. A second auxiliary view that projects the edge view of that surface onto a plane parallel to it will show that surface true size.

To identify oblique surfaces in given orthographic views, realize that the oblique surface will appear as an area in all principle views. The area must contain the same number of points in each view. Unlike the foreshortened views of inclined surfaces, the general shape of oblique surfaces will not maintain similarity in the principal orthographic views.

When given two views of an object, it is usually easy to identify surfaces which match up point for point. If a surface is identified and the points are labeled, project it into the remaining view to see if in fact the surfaces match up point for point. Once the two views of the surface are identified and labeled, the points can easily be projected into the missing view.

If you look at the handout entitled Orthographic Reading of Plane Surfaces, you will notice that inclined surfaces and oblique surfaces can appear as areas in two principal views. When reading a drawing, if you locate a surface that appears as an area in two views, often it can not be determined immediately to be a inclined or an oblique surface. That determination is not crucial. If the areas in the two views are labeled point by point and projected into the missing view, the type of surface will be obvious. If the surface is inclined, it will appear as a line in the missing view. If the surface is oblique, it will appear as an area.