@csk@mathstodon.xyz The following toot helped me figure it out partially: https://mathstodon.xyz/@Danpiker/116289884555776002 You need a tiling of the plane by "anchor" quadrilaterals, where an anchor quadrilateral is a quadrilateral Q such that there is a continuum of squares whose each side passes through a vertex of Q. Now I'm not sure yet what characterizes anchor squares... @Danpiker@mathstodon.xyz