A window with the pivot axis located in the middle of the side stiles of the sash, rotating around a horizontal axis. Typically, the upper sash opens inward towards the room, while the lower sash opens outward. The inward opening part occupies indoor space, so the sash should not be too tall or wide. Designers can arrange several sashes vertically or horizontally as one panel, with the inward-facing lower sash section being taller than a person. Structurally, they are divided into two types: frame-mounted and frame-receiving.

Commonly used for staircases, hallway high windows and transoms, as well as side windows in industrial buildings or ventilating windows. Transmission devices should be installed for larger areas, with common types including rope pull, worm gear, chain drive, and electric.

Center-hung window glass has less loss, rain is less likely to enter, easy to manage, and can also serve as sunshade when frosted glass is installed.