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Why does an oblique surface need a second auxiliary view to show its true shape?

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Figure 1: Two techniques of representing an object. (A) Perspective drawing, suggesting that the object is cubical. (B) Orthographic top and front views, revealing that the object is not cubical. — drafting Figure 1: *Two techniques of representing an object.* (A) Perspective drawing, suggesting that the object is cubical. (B) Orthographic top and front views, revealing that the object is not cubical.

In technical drafting, an oblique surface is defined as a plane that is not perpendicular to any of the three principal planes of projection (horizontal, vertical, or profile). Because it is not parallel to these planes, its true size and shape cannot be captured in a standard principal view or even a single auxiliary view.1

A first auxiliary view can show an oblique surface as a line (an edge view), but it does not reveal the surface's true dimensions or angles. To represent the true size and shape of an oblique surface, a second auxiliary view is required. This process involves an extension of the procedure used for the first auxiliary view, projecting the surface onto a plane that is parallel to the oblique surface itself.1