Why x on horizontal line only?

Answer 1

The x on the horizontal line is used as a reference point. You have a grid. You have a horizontal line and a vertical line. You have two reference points, your x axis and your y axis. In algebra, you search for the point or points where your function crosses the x axis. In calculus, you search for the points where your function changes direction. In advanced geometry, you search for the slope for the slope for the function. As far up or down as you can go, each place on your paper is above or below one point of the horizontal line and can given an x address referring to a point on that line. if you are a million lines out on the x line, your location is 1,000,000x. Your Y line gives you an address as to how high you are above or below the x line. if you are two lines above the x line your y address is 2y. So at that point your function has an address of 1,000,000x, 2y. Everyone knows where the function exists at that point.