Pivot windows have operable sashes that open on a central horizontal axis. The upper half opens inward, and the bottom half opens outward.
The pivot window was patented several times between 1907-1912, in Europe (Austria, Sweden and Germany) as well as in North America. The coupled sash pivot window was patented by Swede Eric S. Persson in 1938.
Pivot windows have been used in Denmark in the late 1930s. Their use became widespread with the flat blocks of the 1970s, since they can be cleaned from within.