Tilings with the neighborhood property

The neighborhood N ( T ) of a tile T is the set of all tiles which meet T in at least one point. If for each tile T there is a different tile T 1 such that N ( T ) = N ( T 1 ) then we say the tiling has the neighborhood property (NEBP). Grünbaum and Shepard conjecture that it is impossible to have a monohedral tiling of the plane such that every tile T has two different tiles T 1 , T 2 with N ( T ) = N ( T 1 ) = N ( T 2 ) . If all tiles are convex we show this conjecture is true by characterizing the convex plane tilings with NEBP. More precisely we prove that a convex plane tiling with NEBP has only triangular tiles and each tile has a 3-valent vertex. Removing 3-valent vertices and the incident edges from such a tiling yields an edge-to-edge planar triangulation. Conversely, given any edge-to-edge planar triangulation followed by insertion of a vertex and three edges that triangulate each triangle yields a convex plane tiling with NEBP. We exhibit an infinite family of nonconvex monohedral plane tilings with NEBP. We briefly discuss tilings of R 3 with NEBP and exhibit a monohedral tetrahedral tiling of R 3 with NEBP.