Relative coordinate system: 
While drawing a line, this system uses a set of new points based on the last point designated rather than (0,0) point of the drawing area.
This is called a relative coordinate system because the coordinate used is relative to the last point used specified.
If the first point of a line is located at (4,6) and you want the line to extends 8 units to the right, the coordinate that is relative to the first point is (8,0), while the actual coordinate of the second coordinate is (12,6).