Third angle projection is the arrangement of orthographic views used on US engineering drawings, and first angle is the arrangement used across much of Europe and Asia. They place the same views in different positions, so a drawing read in the wrong convention produces a mirrored part that passes every dimensional check and does not fit.
How the views are arranged
In third angle the viewer is between the object and the projection plane, so each view is placed on the side of the front view from which it is seen: the right hand view goes on the right, the top view goes above. In first angle the object sits between viewer and plane, so the views swap: the right hand view goes on the left and the top view goes below.
The symbol and where it lives
Both conventions are indicated by a small truncated cone symbol in or near the title block, shown in two views. In third angle the small circle appears to the left of the tapered outline; in first angle it appears to the right. Every drawing should carry it, and a drawing that does not is a drawing that will eventually be misread by somebody.
Why it produces mirrored parts
Because the projection convention decides which side of the front view a feature is drawn on, reading a first angle drawing as third angle puts left hand features on the right. On a symmetric part nobody notices. On a bracket with a hole nearer one edge, or a housing with a boss on one side, the result is a mirror image that measures correctly and assembles into nothing.
Working across conventions
State the convention on every drawing, including the symbol. Where files move between suppliers in different regions, send the solid model as well, because a model has no handedness ambiguity. And when a supplier queries a view arrangement, treat it as a serious question rather than a formality, because the alternative is discovering it after parts are made.
Questions people ask about third angle projection
What is third angle projection?
The orthographic view arrangement used on US drawings, in which each view is placed on the side of the front view from which it is seen.
How do I tell which convention a drawing uses?
By the truncated cone symbol near the title block. In third angle the small circle is on the left of the tapered outline; in first angle it is on the right.
What happens if I read it in the wrong convention?
You get a mirrored part. It measures correctly against every dimension and does not fit, which is why the symbol is not a formality.
Does the model help?
Yes. A solid model carries no handedness ambiguity, so sending the model alongside the drawing removes the risk entirely.