In his book, Linkages: The Different Forms and Uses of Articulated Links, DeRoos stated the following theorem: “In any quadrilateral whose diagonals may intersect at an angle, a point A of a diagonal can always be taken so related to two of the sides that O, A, and B shall be in a line and so that the product of OA and BA shall be a constant.” DeRoos did nothing more in his book, however, than merely state the theorem. He offered no proof of its truth and gave no specifications for the construction of such a linkage. In searching through the literature on linkages, the writer has been unable to find any further mention of this theorem.