In geometry, a tetrad is a set of four simply connected disjoint planar regions in the plane, each pair sharing a finite portion of common boundary.
It was named by Michael R. W. Buckley in 1975 in the Journal of Recreational Mathematics.
[2] However, their placement surrounds an uncovered hole in the plane.
Among solutions without holes, the ones with the fewest possible sides are given by a hexagon identified by Scott Kim as a student at Stanford University.
[2] Gardner offered a number of polyform (polyomino, polyiamond, and polyhex) solutions, with no holes.