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