Decagon n. (Geom.) A plane figure having ten sides and ten angles; any figure having ten angles. A regular decagon is one that has all its sides and angles equal.
... there are six regular polygons containing from five to ten sides, i.e., the pentagon, hexagon, heptagon, octagon, nonagon, and decagon. (Fig. 20.) ... — Dr. Montessori's Own Handbook • Maria Montessori
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