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conjugate diameters[′kän·jə·gət dī′am·əd·ərz]
Two diameters of a conic are said to be conjugate if each of them bisects the chords of the conic that are parallel to the other. Conjugate diameters play an important role in the general theory of conies. In the case of a parallel projection of an ellipse into a circle, conjugate diameters of the ellipse are projected into a pair of mutually perpendicular diameters of the circle.