# Translations:Rays/8/en

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To simplify the mathematics in the rest of this chapter, we will consider only two spatial coordinates: the horizontal coordinate *x* and the depth coordinate *y*. We can use this simplification because it is just a mathematical formality to add the third coordinate, and there is no reason to carry all that baggage along the way. When we do the mathematics, we plot *y* in the upward direction so that we stay within the first quadrant. We learned mathematics in the first quadrant, so we should do mathematics in the first quadrant whenever possible. However, when the final picture is displayed on the computer, then depth *y* would be plotted in the downward direction.