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This article is about technical drawings. See Orthographic projection (geometry) for linear transformations and Orthographic projection (cartography) for maps. For the language concept, see orthography. Orthographic projection is a means of representing a three-dimensional object in two dimensions. It uses multiple views of the object, from points of view rotated about the object's center through increments of 90°. Equivalently, the views may be considered to be obtained by rotating the object about its center through increments of 90°.
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 views may be thought of as being projected onto planes that form a transparent "box" around the object:

Contents [hide] 1 First-angle projection 2 Third-angle projection 3 Additional information 4 Multiviews without rotation 5 Pictorials 6 See also

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[edit] First-angle projection

In first-angle projection, each view of the object is projected in the direction (sense) of sight of the object, onto the interior walls of the box; that is, each view of the object is drawn on the opposite side of the box:
A two-dimensional representation of the object is then created by "unfolding" the box, to view all of the interior walls:
This produces two plan views and four side views.

[edit] Third-angle projection

In third-angle projection, each view of the object is projected opposite to the direction (sense) of sight, onto the (transparent) exterior walls of the box; that is, each view of the object is drawn on the same side of the box:
A two-dimensional representation of the object is then created by unfolding the box, to view all of the exterior walls.

[edit] Additional information

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. Second-angle and fourth-angle projection also are defined, but do not result in useful images.
First-angle projection is as if the object were sitting ON a surface and, from the "face" (front) view, it is rolled to the right to show the left side or rolled over to the left to show its right side; rolled "up" on its back to show its bottom or rolled down on its face to show its top.
Third-angle projection is often considered to be more intuitive than first-angle projection. It is as if the object were a box to be unfolded, or as if it were behind a glass surface and rolled on the far side of the glass to the left to show its left side or rolled to the right to show the right side; or "up" to show the top or "down" to show the bottom.
While Third-Angle projections are prevalent in the USA and Canada, First-Angle projection is more popular in Europe and Asia.
A great deal of confusion has ensued in drafting rooms and engineering departments when drawings are transferred from one convention to another.