# directrices of an ellipse

Directrices of an Ellipse

Two parallel lines on the outside of an ellipseperpendicular to the major axis. Directrices can be used to define an ellipse. Formally, an ellipse is the locus of points such that the ratio of the distance to the nearer focus to the distance to the nearer directrix equals a constant that is less than one. This constant is the eccentricity.