Very strong geometry going on here.. Three rectangles and two opposite triangles, all pointing to an offset center: the construction over the bridge where X marks the spot - or in this case XXX. This litterally stinks of math, but trying to figure out which which rule(s) it bends took me here:
http://www.cabri.net/abracadabri/GeoNonE/GeoHyper/KBModele/Biss3KB.html