braced square

The braced square problem asks, given a hinged square composed of four equal rods (indicated by the red lines above), how many more hinged rods must be added in the same plane (with no two rods crossing) so that the original square is rigid in the plane? The best solution known (left figure above), uses a total of 27 rods (including four for the square), where A, B, and C are collinear (Gardner 1964; Gardner 1984; Wells 1991; Fredrickson 2002, p. 70, Fig. T4).

hinged tessellation | Laman graph | matchstick graph | regular polygon | rigid graph | square | unit-distance graph