O. Aichholzer, M. Kerber, I. Talata, and B. Vogtenhuber
A planar
monohedral tiling is a decomposition of

into
congruent
tiles. We say that such a tiling has the
flag
property if for each triple of tiles that intersect pairwise, the three
tiles intersect in a common point. We show that for convex tiles, there exist
only three classes of tilings that are not flag, and they all consist of
triangular tiles; in particular, each convex tiling using polygons with

vertices is flag. We also show that an analogous statement for the
case of non-convex tiles is not true by presenting a family of
counterexamples.