Glass louvers that overlap one another form the panes of a jalousie window. Operated with a crank or turn-screw, the glass louvers tilt to open, permitting air flow. Besides the fact that they do not obstruct the view, their biggest advantage is they allow ventilation. A wall of jalousie windows can be opened to let breezes flow, making the indoors feel like outside. This advantage however is also their greatest disadvantage -- they allow ventilation so well they are almost impossible to seal. When closed, each glass louver rests against the one below it, rarely if ever making an air-tight seal, and the hinges along the sides are almost impossible to seal without covering the entire window. They are not energy efficient and may be a security risk. Many building codes no longer allow jalousie windows.