In orthographic projection uses two main planes,called reference planes or principal planes of projection.One is horizontal plane(H.P.)and other is vertical plane(V.P.).These two planes intersects each other at right angles in a line XY called as reference line.The projection on the vertical plane (V.P.)is called the front view or the elevation of the object and the projection on the horizontal plane(H.P.)is called the top view or the plane.These two planes form four quadrants.A point may be situated in any of these four quadrants or may lie in one or both of them.
FIRST ANGLE PROJECTION
When we assume the object in the first quadrant i,e. object to be situated infront of the V.P. and above the H.P.the projection called first angle projection.
THIRD ANGLE PROJECTION
When we assume the object in third quadrant i,e.top view above and front view below the reference line,the projection called third angle projection.