The figure shows a parabola with its focus at (0, 1) and a directrix of . Equally spaced concentric circles with their center at the parabola’s focus enable you to measure distances from the focus. Equally spaced horizontal lines parallel to the parabola’s directrix enable you to measure vertical distances from the directrix. This type of graph paper is called focus-directrix graph paper.
Notice that P is the point of intersection of the circle centered at (0, 1) with a radius of 6 and the horizontal line 6 units above the directrix. Thus, P is equidistant from the focus and directrix. Examine the figure and note that all points on the parabola are equidistant from the focus and the directrix.