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 move 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.
Varying widely in size, large sliding windows can effectively function as doors or entryways. Sliding windows of all sizes are common around the world, and may be used in both residential and commercial applications. They are simpler and probably also an older design than sash windows.
In fact, sliding windows likely provided the basis for the vertical design, which became popular in Europe in the 16th century, after the development of a counterweight system made them viable. Both vertical and horizontal designs, as they are known in the 21th century, were refined in England, and have changed little in their basic form for several hundred years. In some places, sliding windows are still referred to as Yorkshire sash windows, in reference to the British town where they originated.
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. More expensive versions sometimes tilt in or out, in addition to sliding back and forth. This allows for even greater modulation, for venting air.
Other optional features common on sliding windows are a variety of frame finishes, insect screens, wooden grilles, and different kinds of glass. Optional finishes on the glass can be UV-protective, tinted, or shaded in some way to cut down on light and heat. The glass may also be tempered for extra strength, or be double-paned to improve insulation.
Though the most common color of sliding window frames is white, this was not always the case. In fact, in the 18th and 19th centuries, white was never used, and darker shades of red, green, or brown stains were the norm. These colors served to camouflage the wood and give the impression that larger panes of glass were used, which were more expensive and therefore more desirable.
Today, there is a fairly large industry, both the United States and Europe, of companies that specialize in recreating Yorkshire sash windows for use in historic homes. These businesses combine period materials and techniques with modern ones to produce windows that look vintage, but have modern touches, like double-glazing. While this style of window is expensive, for both sentimental and historic reasons, many owners of older homes choose to incorporate these designs to maintain an original appearance.