CourseWiki/Engineering Graphics and CAD/Reading Drawings

First-Angle and Third-Angle Projection

Identify the projection symbol before interpreting where the top and side views sit on the sheet.

Check the projection convention before naming a side view

First-angle and third-angle projection can show the same part from the same directions. What changes is where those views are arranged on the sheet.

Look for the projection symbol or an explicit note, often near the title block. Use it with the view labels. If the symbol and the views disagree, resolve the inconsistency before interpreting the part.

View placement relative to the front view
Named view	Third-angle	First-angle
Top	Above	Below
Bottom	Below	Above
Right side	To the right	To the left
Left side	To the left	To the right

A right-side view is always the view from the part's right. In first-angle projection, it sits to the left of the front view. Do not rename it a left-side view because of its position.

The projection symbol is two views of a shortened cone

The trapezoid is the side view of a cone with its tip removed, called a frustum. The two concentric circles are its end view. Their arrangement tells you which projection method is being used.

Projection symbols with the frustum's small end on the left: third-angle has the circular end view left of the trapezoid; first-angle has it on the right. Below, third-angle places top above front and right at right; first-angle places top below front and right at left. — Read the circle's position together with the taper direction. These examples hold the frustum orientation fixed so the difference is easy to see.

Use the glass box to explain the swap

In third-angle projection, the viewing plane is between the observer and the object. In first-angle projection, the object is between the observer and the viewing plane. Unfolding those planes places the named views on different sides of the front view.

The angle names refer to projection quadrants. They do not mean rotating the part by one degree or three degrees, and they do not specify the number of views.

Compare the primary teaching diagrams in NPTEL's first- and third-angle lesson. Autodesk's drawing documentation also explains how the selected convention controls projected-view placement.

Expect third-angle on U.S. mechanical drawings, then verify it; Practice

Expect third-angle on U.S. mechanical drawings, then verify it

Third-angle is the usual U.S. mechanical drawing convention. A shared convention lets the designer and the person making or inspecting the part interpret the layout the same way. It is a communication choice; neither method changes the part's geometry.

The drawing's declared convention is the deciding information. Country of origin, inch units, or a familiar company name is not a substitute for checking. Drawings cross borders, and CAD templates can use different defaults.

A documented U.S. example, with its scope

The NASA Goddard Engineering Drawing Standards Manual (1994), §3.1, printed p. 25 states: Third-angle orthographic projection shall be used for mechanical engineering drawings. That is a requirement in that manual, not proof that every U.S. drawing uses third-angle. Autodesk's Fusion documentation gives a current software example: its ASME drawing setting uses third-angle projection. Bricsys's projection-type reference likewise identifies third-angle with U.S. practice.

Use the view explorer to switch layouts while holding the part fixed. Then work through the projection self-check.

Appears in

DET 102 DET Reading Engineering Drawings