First of all, we know that the first angle projection system and third angle projection system both are the methods used for orthographic projection drawing.
In the first angle projection system, the object placed in the first quadrant and in third angle projection system the object placed in the third quadrant.
If one regards the artesian coordinates as being made of quadrants, then 1st angle and 3rd angle are two different quadrants in which an object is places and different planes on to which the drawing is projected.
In 1st angle, the object is between the observer and the plane of projection.
In 3rd angle, the plane is between the observer and the object.
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First Angle Projection
In this, the object is assumed to be positioned in the first quadrant and is shown in figure. The object is assumed to be positioned in between the projection planes and the observer. The views are obtained by projecting the images on the respective planes.
Note that the right-hand side view is projected on the plane placed at the left of the object. After projecting on to the respective planes, the bottom plane and left plane is unfolded on to the front view plane. i.e. the left plane is unfolded towards the left side to obtain the Right-hand side view on the left side of the Front view and aligned with the Front view. The bottom plane is unfolded towards the bottom to obtain the Top view below the Front view and aligned with the Front View.
Third Angle Projection –