Each point on the locus satisfies this equation. Conversely, by reversing the steps, it can be shown that every point which satisfies this equation fulfills the conditions of the locus. We call this the equation of the parabola with focus at (p/2,0) and with the line x = -p/2 as directrix. The vertex of the parabola is at the origin. It is apparent from the equation that the x axis is an axis of symmetry for if y is replaced by -y, the equation is unchanged. The line of symmetry of a parabola is called the axis of the parabola.