The 3D cursor is supposed to lie in the world 3D space, and appears on the screen superposed on the background image. Each of the four sides can be dragged parallelly within underlying plane. Also it can be rotated any amound of angle around one of its sides.
    This is the upper half of a building. The red rectangular shape, which consists of four red lines is the 3D cursor. On the right side, there are two rectangles already created. They are rendered in translucent grey.
    The left side of the 3D cursor is dragged so that it fits to the edge of the wall. A rectangle is created there as shown in the next figure.
    The 3D cursor is rotated around the left side. It is rotated 90 degree by default. 
    A side of the 3D cursor is dragged again to the left edge of create another rectangle...