The simplist form of projection, is to simply allong lines parallel to the z-axis. This form of projection is called orthographic or parallel. It is the common form of projection used by draftspeople for top, bottom, and side views. The advantage of parallel projection is that the you can make accurate measurments of image features in the two dimensions that remain. The disadvantage is that the images don't appear natural (i.e. they lack perspective foreshorting).

Here is an example of an parallel projection of our scene.

Notice that the parallel lines of the tiled floor remain parallel after orthographic projection.