To understand why 2nd and 4th angle projection is not used? First we need to understand the concept of orthographic projection system.

Theoretically there are four projection system, out of which only 1st and 3rd angle projection system are used because of limitation of 2nd and 4th angle projection system.

**How Projection System Works**

To understand how projection system works We can consider a point in 1st and 3rd

quadrant (ref. image on right)

Consider **Point A** in 1st quadrant that lies in between observer and projection plane. Front view of point A will be in vertical plane(V.P.) and top view will be on horizontal plane. To get front and top view in 2D plane, as per rule, horizontal plane need to be rotated by 90 degree in clockwise direction.

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