Tangents at parameters t1 and t2 on $y^2$ = 4ax meet at the point (at1t2, a(t1+t2)). This is a key result: the x-coordinate is a times the product and the y-coordinate is a times the sum. For a focal chord (t1t2=-1), the tangents meet at (-a, a(t1+t2)), which lies on the directrix x=-a. The tangents at the ends of a focal chord are perpendicular.