Window Terms Explained: A Basic Glossary
The glossary of terms that follows provides some of the common terms associated with windows that you may encounter when reviewing companies and products.
Apron: A piece of horizontal sash or window trim applied against the wall immediately below the stool; serves to conceal the joint between the window frame sill and the plaster on interior finish surface.
Basement Sash Unit: (cellar sash unit) - A sash unit, usually of the in swinging or hopper sash type, used for basement or cellar sash openings.
Bay Window: Composed of three or more individual windows, generally with the side or flanker units at 45° or 30° angles to the wall of the structure. Bottom Rail: The bottom horizontal member of a window sash.
Bow Window: Composed of three or more individual windows in a gently curved contour. Bow windows also project from the wall of the structure.
Cap/Capping: Cosmetic covering, usually found on the exterior of the window to achieve aesthetic sight lines.
Casing (Trim): Exposed molding or framing around a window, on either the inside or outside, to cover the space between the window frame or jamb and the wall.
Circle Top Window: A window having a curved top and a flat bottom. The shape of the window is a half-circle with the height being usually one-half of the width.
Cottage-Style Window: Hung Window (Single or Double) that has a larger bottom sash (lite) than the top sash (lite).
Desiccant: A drying agent usually in granular form used by some manufacturers between the panes of insulating glass to prevent “fogging” between the panes.
Double Hung Window: Two sash which move vertically, by-passing each other in a single frame. Sash may be counter-balanced by weights or springs.
Double Window: Two windows separated by a mullion, forming a unit. Also called a twin window unit.
Eyebrow Window: Today, used to identify certain arch-topped or radius-topped windows that have a curved top like the shape of a human eyebrow.
Fixed Light (Fixed Sash): Window, which is non-operative (does not open).
Frame: Outside member of a window which encloses the sash.
Gliding Window: Same as a sliding window, the moving sash travels on rollers.
Hopper Vent: Inward-opening sash hinged at the bottom (an upside-down awning window). Many basement windows are this type.
Hung Window: Window with one or more hanging (counter balanced) sashes.
Jamb: A vertical member at the side of the window frames; also refers to the horizontal member at the top of the window frame (Head Jamb and Window Jamb).
Keeper: The part of a window lock, mounted on an opposing surface of the window, that the lock arm locks under or into to pull the sash into a locked position and fully releases it when opened.
Latch (Catch/Lock): A device which holds a window shut, such as the latch at the meeting of a double-hung window, often referred to as Lock.
Lite (light): A window; a pane of glass within a window. The number of lites in upper and lower sash designates double-hung windows, as in six-over-six. Often spelled “Lite” to differentiate from sunlight or other light sources that shine through a window.
Low - E Glass: Glass that has been given a special micro-thin coating that blocks the passage of radiant heat through the glass for better energy efficiency without appreciably affecting the view through the glass (like tinting can do).
Obscure Glass: A glass (frosted, etched, fluted, ground, etc.) for privacy, light diffusion, or decorative purposes.
Picture Frame Casing: The use of trim casing on all four sides of the interior of a window, resembling a picture frame when installed.
Pocket Sill: A deep sill (frame) design that completely surrounds the bottom edge of the sash in the closed position.
Prime Sash: The balanced or moving sash of a window unit.
Quarter Round Window: Stationary or operating window with glass shaped as a quarter circle.
Radiation: Energy released in the form of waves or particles, due to a change in temperature within a gas or vacuum.
Rail: Horizontal members of a window sash or door panel.
Roof Window: Sloped application of fenestration product, which allows for in-reach operation and rotation of sash to facilitate the cleaning of exterior surfaces. R-Value (Thermal Resistance): A measure of the resistance a unit of heat has in flowing through a given material or construction; a higher value indicates a better heat-insulating property.
Sash: A single assembly of stiles and rails in which the lites of a window are set. The framework holding the glass in a window unit.
Side Lite (light): A fixed, often narrow, glass window next to a window.
Sill: The horizontal member at the bottom of the window frame; a masonry sill or sub-sill can be below the sill of the window unit.
Single-Hung Window: A window that is similar to a double-hung window except that the top lite (light) is fixed (stationary).
Stop: A trim member attached to the window frame to stop the sash of a projecting window when closed to prevent it from swinging through the opening.
Tempered Glass: Special heat-treated, high-strength safety glass which shatters into pebble-sized particles but not into long slivers, when broken.
Ultra-Violet: Type of radiation with wavelengths shorter than those of visible light and longer than those of X-rays. Causes sunburn, fading and breakdown of fabric, wood, furniture and other exposed surfaces.
Vertical Sliding Window: One or more sashes that move in a vertical direction.
Warm Edge Spacer: Use of a non-conductive edge spacer in insulating glass units instead of the conventional metal (conductive) edge spacer.
Weephole: Small holes drilled along the bottom edge of storm sash, combination storm-screens, or windows with “pocket” sills to permit moisture condensation or wind-driven rain to drain away from the sill to the outdoors.
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