The ellipse and the hyperbola have an additional axis of symmetry, perpendicular to the first, so there is an alternate focus and directrix F' and d' obtained as the reflection of F and d with respect to this axis. The simplest analytic form for the ellipse and hyperbola is obtained when the two symmetry axes coincide with the coordinate axes. The distance from the center to either focus is , and the sum of the distances from a point in the ellipse to the foci is 2 a . The segment thus determined, and its length 2 a , is also called the transverse axis, while the length 2 b is also called the conjugate axis. The distance from the center to either focus is , and the difference between the distances from a point in the hyperbola to the foci is 2 a .
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