Editing Engineering drawing (section)

==== Multiview projection ====
A ''[[multiview projection]]'' is a type of [[orthographic projection]] that shows the object as it looks from the front, right, left, top, bottom, or back (e.g. the ''primary views''), and is typically positioned relative to each other according to the rules of either [[Multiview projection|first-angle or third-angle projection]]. The origin and vector direction of the projectors (also called projection lines) differs, as explained below. 
* In ''first-angle projection'', the parallel projectors originate as if radiated ''from behind the viewer'' and pass through the 3D object to project a 2D image onto the orthogonal plane ''behind'' it. The 3D object is projected into 2D "paper" space as if you were looking at a [[radiograph]] of the object: the top view is under the front view, the right view is at the left of the front view. First-angle projection is the [[ISO 128|ISO standard]] and is primarily used in Europe.
* In ''third-angle projection'', the parallel projectors originate as if radiated ''from the far side of the object'' and pass through the 3D object to project a 2D image onto the orthogonal plane ''in front of'' it. The views of the 3D object are like the panels of a box that envelopes the object, and the panels pivot as they open up flat into the plane of the drawing.<ref name="French_Vierck_1953_pp99-105">{{Harvnb|French|Vierck|1953|pp=99–105}}</ref> Thus the left view is placed on the left and the top view on the top; and the features closest to the front of the 3D object will appear closest to the front view in the drawing. Third-angle projection is primarily used in the United States and Canada, where it is the default projection system according to [[ASME]] standard ASME Y14.3M.

Until the late 19th century, first-angle projection was the norm in North America as well as Europe;<ref name="French1918p78">{{Harvnb|French|1918}}, [https://books.google.com/books?id=6R5DAAAAIAAJ&pg=PA78 p. 78].</ref><ref name="French_Vierck_1953_pp111-114">{{Harvnb|French|Vierck|1953|pp=111–114}}</ref> but circa the 1890s, third-angle projection spread throughout the North American engineering and manufacturing communities to the point of becoming a widely followed convention,<ref name="French1918p78"/><ref name="French_Vierck_1953_pp111-114"/> and it was an ASA standard by the 1950s.<ref name="French_Vierck_1953_pp111-114"/> Circa World War I, British practice was frequently mixing the use of both projection methods.<ref name="French1918p78"/>

As shown above, the determination of what surface constitutes the front, back, top, and bottom varies depending on the projection method used.

Not all views are necessarily used.<ref name="French_Vierck_1953_pp97-114">{{Harvnb|French|Vierck|1953|pp=97–114}}</ref> Generally only as many views are used as are necessary to convey all needed information clearly and economically.<ref name="French_Vierck_1953_pp108-111">{{Harvnb|French|Vierck|1953|pp=108–111}}</ref> The front, top, and right-side views are commonly considered the core group of views included by default,<ref name="French_Vierck_1953_p102">{{Harvnb|French|Vierck|1953|p=102}}.</ref> but any combination of views may be used depending on the needs of the particular design. In addition to the six principal views (front, back, top, bottom, right side, left side), any auxiliary views or sections may be included as serve the purposes of part definition and its communication. View lines or section lines (lines with arrows marked "A-A", "B-B", etc.) define the direction and location of viewing or sectioning. Sometimes a note tells the reader in which zone(s) of the drawing to find the view or section.