Many solid or three-dimensional pieces of scenery are regular architectural forms, such as steps, columns, newel posts, and the like, and they are very easily explained in the orthographic technique. The usual front, side, and top views give the carpen- ter all the information he needs. However, some pieces have slanted sides or are angled, either to increase the perspective or to distort them for design purposes. Whatever the reason, there is the problem of drawing a surface that is inclined to two, or more, of the principal planes of projection. The regular front, side, and top views do not show the inclined surface in true dimension.
Carrying the orthographic technique a step further, the observer is free to move around the object to the angle that allows a view straight at the slanted surface. From this perpendicular view, the surface is seen in true shape. Because it is not one of the normal views, it is known as an auxiliary view. The process of getting an auxiliary view onto the drawing board requires the use of a new plane of projection, a reference plane, and a generous amount of visual imagination.
At a position opposite the auxiliary face, there is imagined a plane that is parallel to the slant of the face and containing a projection of the surface in true shape. The plane is not one of the principal planes of projection, so it is referred to as an auxiliary plane. It is usually perpendicular to one of the principal planes of projection and it is slanted to the other two. This means that in one of the three views, the slanted surface is seen from the edge, or in profile, and it appears as a line. The auxiliary plane, being parallel to the auxiliary face, also appears as a line and is rotated onto the plane of the paper to reveal the projection of the true shape.
The hinge of the rotation is the intersection of the auxiliary plane and the reference plane (a second imaginary plane that is always located perpendicular to the auxiliary plane and, consequently, is usually parallel to one of the principal planes of projection). The reference plane, like the base line, is used to establish distances and the location of points on the slanted surface. The reference plane always appears as a line because it is only seen in edge view, or as it intersects the auxiliary plane. The auxiliary plane, of course, is only shown in its rotated position revealing the projection of the auxiliary face.
Because the edge view of the slanted surface may occur in any one of the principal views, an auxiliary view may be