Most basically, a window is an opening in a wall or roof that allows the transfer of light and sometimes air. Initially windows were simple holes that were then filled in with cloth or animal hides, eventually replaced as time went on by glass and then glass separated into panes. Today, there are many important factors to consider when selecting a window.

Windows come in a variety of styles:

• Hung, single with one sash moving (usually the bottom sash) and double with both sash moving.
• Awning, hinged at the top and opening outward.
• Bay, made up of three or more windows with the center window being parallel to the building wall and the side windows at 30, 45 or 90 degrees to the wall. Typically the center is stationary and the flankers are hung.
• Bow, a series of four or more window installed as a radius out from the building wall.
• Fixed, commonly referred to as a picture window.
• Garden, a window extending outward typically used for plants with side ventilation.
• Slider, one or both sash move horizontally.
• Casement, hinged on the side and swings outward like a door.
• Transom, a window above a door, usually fixed but may be hinged.
• Skylight, in a flat or sloped roof and may be fixed or venting.
• Roof window, typically built into a sloped roof and may provide egress.

Windows can be a significant source of heat transfer. As such, there are several considerations to weigh in choosing the type of window for installation in a structure. One important factor is the frame material — wood, metal, vinyl or composite. Each has different structural, thermal and sustainability properties.

Glass and glazing are also important considerations. Specifically, be sure to address the number of glass layers, inert gas in between the layers, coatings for thermal and light spectrums permitted and sealants.