This is a 4th dimensional Perspective coordinate plane made up of 4 axes (s,x,y,z) The Shadow-Axis represents the Within and Without directions The x-axis represents the Left and Right directions The y-axis represents the Forewords and Backwards directions The z-axis represents the Up and Down directions So your probably wondering what is the shadow-axis well the shadow axis is a derivative of the w-axis, the w-axis which is found at a 90 degree angle of all axes in the 3rd dimension. This is impossible to represent in perspective because the w-axis would just point in the origin. This can also be found in the 2nd dimension with the z-axis at a 90 degree angle of all the axes as it too will point in the origin. the solution to this other then doing vary high level mathematics is instead of a 90 degree angle of all axis you can do a 45 degree angle of all axes which is also coincidentally the shadow of the w-axis. this type of projection is called: Orthographic Perspective