A chord which is a normal at each end. A centrosymmetric set has double normals through the center (Croft et al. 1991). For a curve of constant width, all normals are double normals.

Croft, H. T.; Falconer, K. J.; and Guy, R. K. Unsolved Problems in Geometry. New York: Springer-Verlag, p. 15, 1991.

Kuiper, N. H. "Double Normals of Convex Bodies." Israel J. Math. 2, 71-80, 1964.

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Weisstein, Eric W. "Double Normal." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/DoubleNormal.html