Third Angle projection is a method of orthographic projection which is a technique in portraying a 3D design using a series of 2D views.
The Third Angle Projection or First Angle Projection, different countries and regions typically use one method or the other although this can depend on a company’s customer.
Since World War 2 where the UK sent engineering drawing to the US for manufacture the UK has adopted 3rd Angle projection as the general standard. The US, Canada and Australia are the only other regions that typically use 3rd Angle projection as standard.
3rd Angle project is where the 3D object is seen to be in the 3rd quadrant. It is positioned below and behind the viewing planes, the planes are transparent, and each view is pulled onto the plane closest to it. The front plane of projection is seen to be between the observer and the object.
The images below show the projection of the object on a 3D box surrounding the object. The box is then gradually unfolded to then present a series of 2D views in the 3rd Angle projection as viewed by the observer.
The following demo shows this in motion:
The views below show the same object in first an Isometric 3D view then the corresponding 2D 3rd Angle projection views in the specific alignment. The annotations on the 2D views show how the Top and Left views are aligned to the front view.
The 3rd Angle projection symbol shows the orientation of a cone in the 3rd Angle projection.