The views are positioned relative to each other according to either of two schemes: first-angle or third-angle projection. In each, the appearances of faces may be thought of as being projected onto planes that form a "box" around the object.
In first-angle projection, each view of the object is projected "through" the object, onto the interior walls of the box. A two-dimensional representation of the object is then created by "unfolding" the box, to view all of the interor walls.
In third-angle projection, each view of the object is projected "outward" from the object, onto the (transparent) exterior walls of the box. A two-dimensional representation of the object is then created by unfolding the box, to view all of the exterior walls.
The term "third-angle" is used because, compared to "first-angle" projection, the directions of projection are rotated through two right angles about the object. First-angle projection is often considered to be less intuitive than third-angle projection.
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