A little background for our American friends:
Europeans suffer from excessively air-tight living spaces (while Americans still busily try to figure out how to increase the air tightness). The typical perimeter hardware of an European window allows for untying the window, securing it with a small gap at the top. In this case, it was not enough for the occupant of the space. Please note the mitered corners of the sashes and glazing bars, which allow for continuous perimeter seals. We will see them in America perhaps in ten years.