Projections on Auxiliary Planes
Sometimes none of the three principal orthographic views of an object show the different edges and faces of an object in their true sizes, since these edges and faces, are not parallel to any one of the three principal planes of projection.  In order to show such edges and faces in their true sizes, it becomes necessary to set up additional planes of projection other than the three principal planes of projection in the positions which will show them in true sizes.  If an edge or a face is to be shown in true size, it should be parallel to the plane of projection.  Hence the additional planes are set up so as to be parallel to the edges and faces which should be shown in true sizes.  These additional planes of projection which are set up to obtain the true sizes are called Auxiliary Planes. The views projected on these auxiliary planes are called Auxiliary Views.

The auxiliary view method may be applied

• To find the true length of a line.
• To project a line which is inclined to both HP and VP as a point.
• To project a plane surface or a lamina as a line.

Types of auxiliary planes
Usually the auxiliary planes are set up such that they are parallel to the edge or face which is to be shown in true size and perpendicular to any one of the three principal planes of projection.  Therefore, the selection of the auxiliary plane as to which of the principal planes of projection it should be perpendicular, obviously depends on the shape of the object whose edge or face that is to be shown in true size.

• If the auxiliary plane selected is perpendicular to HP and inclined to VP, the view of the object projected on the auxiliary plane is called auxiliary front viewand the auxiliary plane is called auxiliary vertical planeand denoted as AVP.
• If the auxiliary plane is perpendicular to VP and inclined to HP, the view of the object projected on the auxiliary plane is called auxiliary top viewand the auxiliary plane is called auxiliary inclined planeand denoted as AIP.