Sliding Windows are windows constructed in a frame and installed so that they slide open and closed, as opposed to opening, in or out, on a hinge. Though the type that moves up and down vertically are also technically a form of sliding window, this specific term is generally reserved for those that slide horizontally. Vertically-sliding windows are known simply as sash windows. The most common type of sliding window consists of two panes of framed glass, known as sashes, mounted and installed on parallel rails in a window frame. Generally, each sash can be slid past the other, though some cheaper models have one fixed sash and only one movable one. Other versions sometimes tilt in or out, in addition to sliding back and forth. This allows for even greater modulation, for venting air.