The invention relates to an improved sash window including a window frame (1) containing two sashes (2 and 3) which are disposed in the same first vertical plane in the closed window position. The lower sash (3) is connected to a counterweight mechanism (8) enabling same to move vertically in the window frame (1). The window also includes scissor mechanisms (9) which are guided between the jambs (4) of the window frame (1) in order to move the lower sash (3) horizontally forward from the first vertical plane in which the window is closed to a second vertical plane in which the window is open or vice versa. The lower sash is maintained in the open position by means of blocking elements (24, 25) and is moved vertically with the aid of the counterweight mechanism (8).