The fundaments of the Kuhna Ark, the historical fortress of Khiva date from the 5th century, butr most of the buldings date from the 19th century.[br]The summer mosque dates from 1838. The glazed tiles are created by two local masters: Ibadullah and Abdullah Jin, who also created the tiles of the Tash Hauli palace.[br]Remarkable is that the walls of the iwan of the summer mosque completely are decorated with glazed tiles, but in different patterns.

One of the patterns is a tiling of double bell shapes in a square grid.

[u]Left[/u]: You can draw the bell shapes starting from tangent circles.[br][u]Right[/u]: You can draw the double bell shapes starting from the square grid.[br]The basic rule of a tiling is that you can modify the sides of the square so that an extentions is equally compensated.[br]A modification of one side can be transferred to another side by rotation or translation, forming the unit of the tiling.