The Yff hyperbola is the hyperbola given parametrically by cos A + sin(B - C + t):cos B + sin(C - A + t):cos C + sin(A - B + t). The trilinear equation is complicated expression with coefficients up to degree 10 in the side lengths. This hyperbola has vertices at the triangle centroid G and orthocenter H, a focus at the circumcenter O, and a directrix given by the line passing through the nine-point center N and perpendicular to the Euler line. Its center is therefore the midpoint of G H, which is Kimberling center X_381.