Coupled sash window

Coupled sash windows consist of two independent sashes with a shared hinge on the frame. The sashes are held together with coupling fittings. The system was patented in 1889 by the Swedish architect Carl Flodquist (1858-1915) and the engineer C.G. Hall. Windows with coupled sashes and two layers of glazing provide good insulation. They became common in Northern Europe in the early 20th century.