>> You know we start with a -- in our previous discussions we have a two-dimensional coordinate plane. And we have an x axis and a y axis and they are perpendicular to one another and they are arranged like this where the x axis is horizontal, the y axis is vertical. Well, when we introduced the three-dimensional coordinate system, we need a third axis and it's kind of logical to think that that third axis would go through like this, you see just come out of the middle of the origin right here and the zero point on this number line would be at the origin of the original coordinate plane. Well, actually we are going to rearrange it a little bit and there is some convenience in this rearrangement and we'll see that convenience later, but the way that it's arranged is like this and notice the way that it's illustrated and by the way, the illustrations in your textbook are outstanding. They are much better than I can do on the board, but let me take a shot at on arrangement right here. Notice the z axis is going up here, the y axis is to the right and the x axis is sort of coming at us. Now, to illustrate a number line that's coming at us, the positive values are coming at us, the negative values are on the other side of the board, but to illustrate the idea, we have to draw that x axis somehow so, it's kind of drawn like this. Now, it puts the whole system into a kind of perspective which is not oriented toward right angles and that's what makes the whole study a little bit difficult, a little bit challenging.