Draw a little butterfly on your page in approximately the same position as mine.
But where exactly is the butterfly? How far from us, how high above the ground? We have no idea until we 'place' our butterfly. To do that we shall put her in a (reference) box or cage. See below ...
Now we have our reference and we can relate the position of the butterfly to something tangible, specific. Furthermore if we draw a line 'forward' from our butterfly we can decide where we want our butterfly in relation to the front of our box (point 'a')
In a similar fashion if we project a vertical line up from our butterfly we can relate the butterfly to the top of our box (point 'b'). But how far up? Since the butterfly is in a set place then point 'b' can be joined by the vertical projection and by the projected line to the left vanishing point. It is as if we sliced through our box and the 'plane' of that slice suddenly cut through the middle of our butterfly.
Do this again and locate the butterfly on point 'c' the left hand front
surface. Such is the method we can obtain the 'three dimensions' that
locate our butterfly in space. Of course we needed to 'reference' the
butterfly within a box. Now we have our 'three dimensions'. The bigger
butterfly I have introduced is much closer and can be related to the
first in a similar manner.
Prepare for the next lesson by drawing a 'top view' and 'side view' of a common fork.