Window and Door Glossary (A-Z)

This glossary defines window and door terms as used in the United States, with the European system terms that appear on drawings and submittals. Terms are listed alphabetically. Where a term has its own guide on SeeWindow, the term links to it. Rating values quoted here are scales, not product claims.

A

Air infiltration - Air that leaks through a closed window or door. Lower is better, and it is reported as air leakage on the NFRC label.

Air leakage (AL) - An NFRC label rating for how much air passes through a product. It is one of five ratings on the label, alongside U-factor, SHGC, VT and condensation resistance.

Apron - Trim under the stool of a window, set flat against the wall below the sill.

Area well - A pit in the ground outside a below-grade opening. Under the 2021 IRC (R310.4) an egress area well needs at least 9 sq ft (0.84 m2) with a 36 in (914 mm) projection and width, plus a ladder or steps if deeper than 44 in (1,118 mm). See egress window requirements.

Argon - An inert gas used between the panes of an insulating glass unit. It insulates better than air.

Astragal - The center post of a double door, attached to the fixed or inactive panel and closing against the active one.

ASTM F2090 - The ASTM standard for window opening control devices and fall-prevention devices. The 2021 IRC permits compliant devices on egress windows at no more than 70 in (1,778 mm) above the finished floor.

Awning window - A window hinged at the top that opens outward from the bottom.

B

Bay window - Three or more window units angled out from the wall, usually with a larger center unit.

Bow window - Three or more window units curved outward from the wall in an arc.

Brick mold - Exterior trim traditionally used around a window and as the attachment point to the wall.

C

Casement window - A window hinged on one side that swings open like a door. Compare tilt and turn vs casement.

Casing - Interior molding that covers the gap between the unit and the rough opening.

Check rail - The point where the two sashes of a double-hung window meet, and where the lock is usually mounted.

Cladding - A low-maintenance exterior covering that protects the frame, such as aluminum over wood. See aluminum-clad wood windows.

Clear opening - The open area of a window when fully open. See net clear opening.

Clerestory - A high window, usually out of reach from the floor.

Condensation - Water that forms on a cold glass or frame surface when humid air touches it. See window condensation.

Condensation resistance (CR) - A rating for how well a product resists condensation on its interior surface. Higher is better, and the NFRC label can list it. One manufacturer glossary describes a laboratory scale of 1 to 100.

D

Design pressure (DP) - The wind-load pressure, in pounds per square foot, that a building code requires a window or door to withstand. It is compared with the product's performance grade.

Double-hung window - A window with an upper and a lower sash that slide vertically past each other.

Drip cap - A cover above a window or door, usually aluminum or vinyl, that sheds water away from the unit.

Dual-pane glazing - Two panes of glass separated by a gap, which is gas-filled or air-filled in an insulating glass unit.

E

Egress window - An operable window large enough to serve as an emergency exit. Sizes come from IRC R310; see egress window requirements.

EPDM - Ethylene propylene diene monomer, a synthetic rubber used for window and door gaskets. It resists ozone, UV and weathering and stays flexible across a wide temperature range.

Espagnolette - The handle-operated drive on European sashes that moves the locking points around the sash. It is part of the multipoint lock.

Extension jamb - A piece that extends the frame depth to match the wall thickness.

F

Fenestration - Any opening in a building envelope that holds a window, door or skylight, and the products themselves.

Fixed window - A window that does not open. Also called stationary.

Flashing - Water-resistant material that diverts water away from the window and out of the wall.

Folding door - A door of several hinged panels that fold to one side to open a wide clear space.

Frame - The outer structure that holds the sash or the door panel.

Full-frame window - A window installed with its frame directly into the rough opening. Compare insert window.

French door - A hinged door with a large glass area, wide stiles and a tall bottom rail.

G

Gas fill - An inert gas sealed between the panes of an insulating glass unit, usually argon or krypton, to reduce heat transfer compared with air.

Glazing - The glass in a sash or panel, and the act of installing it.

Glazing bead - A strip of wood or vinyl that holds the glass in the sash and covers the edge.

Gliding door - A sliding door in which two or more panels slide horizontally past each other. Also called a sliding patio door.

Grille - A bar or grid that divides or simulates division of the glass into smaller lites. Also called muntins or dividers.

H

Handing - The direction a hinged unit opens, set by the hinge location when viewed from outside.

Head - The top horizontal member of a window or door frame.

Header - The beam above a rough opening that carries the load around the opening.

I

IGU (insulating glass unit) - Two or more sealed panes of glass held apart by a spacer, often with a gas fill and a low-e coating.

Impact-resistant glass - Laminated glass built to resist flying debris in hurricane-prone areas.

Insert window - A new unit installed inside the frame pocket of an existing window. Compare full-frame.

IRC - The International Residential Code, the model code for one- and two-family houses. States and cities adopt and amend it.

J

Jamb - The frame members that form the top and sides of a unit.

Jamb liner - The interior surface of the frame on which a double-hung sash slides.

L

Laminated glass - Layered glass bonded with an interlayer that holds the pieces together when broken.

Lift and slide door - A sliding door whose handle lifts the sash off its seals onto carriages so it can roll, then lowers it onto the seals when closed. See lift and slide doors.

Lite - One pane of glass in a sash or panel. Also spelled light.

Low-e coating - A microscopically thin, nearly invisible metal or metal-oxide layer on glass that reduces radiant heat transfer. Different coatings are tuned for low or high solar gain.

M

Masonry opening - An opening in brick, stone or stucco where a unit is installed.

Meeting stile - The vertical edge where two sliding sashes or panels overlap in the center.

Mullion - The vertical or horizontal joint between window units that are combined into one assembly.

Multipoint lock - A lock that engages the sash at several points around its perimeter with one motion of the handle. It pulls the sash against its seals evenly.

Muntin - A bar that divides a window into lites. See grille.

N

NAFS - The North American Fenestration Standard, which sets performance requirements for windows, doors and skylights in the US and Canada and defines the performance classes R, LC, CW and AW.

Nailing fin - A flange around the unit perimeter, fastened to the wall framing, that attaches the unit and provides a flashing connection. Also called a nailing flange or installation flange.

Net clear opening - The unobstructed opening of a window in its normal operating position, used to judge egress. The 2021 IRC sets minimums for area, height and width.

NFRC - The National Fenestration Rating Council, an independent nonprofit that certifies the energy performance of windows, doors and skylights. See the NFRC label explained.

O

Obscure glass - Textured or patterned glass that lets light through and limits the view.

OITC - Outdoor-Indoor Transmission Class, a rating of sound reduction between outside and inside. See STC.

OX / XO - Codes for sliding configurations as seen from outside. X is the operable panel and O is the stationary panel.

P

Panel - The rail, stile and glass assembly that forms a door leaf.

Performance class - The NAFS tier of durability, either R, LC, CW or AW, with higher classes meeting stricter test criteria. Class R is the one NAFS suggests for one- and two-family dwellings.

Performance grade (PG) - A rating derived from testing that reflects the structural, air and water performance of a product and is compared with the design pressure.

Pocket door - A door that slides into a cavity in the wall.

R

Rail - A horizontal member of a sash or door panel.

Rough opening - The framed opening in the wall for a window or door, larger than the unit to allow for shimming, flashing and insulation.

S

Safety glazing - Glass such as tempered or laminated glass that is required in hazardous locations, including doors and some glazing near doors, under IRC R308.

Sash - The frame that holds the glass in the part of a window that opens or is fixed within the main frame.

Seal - A compressible surface that blocks air and water at the joint between sash and frame.

Shim - A wedge, often wood, used to level and plumb a unit in the rough opening.

SHGC (solar heat gain coefficient) - A rating from 0 to 1 for how much solar heat a product lets in. Low values resist solar heat, and high values admit it.

Sidelight - A tall, narrow unit beside a door or window.

Sill - The bottom horizontal member of a frame.

Simulated divided lite (SDL) - Grilles applied on the interior and exterior of the glass to resemble divided lites, with no spacer between the panes.

Spacer - The strip between the panes of an insulating glass unit that keeps the panes the correct distance apart. It reduces heat flow and helps limit condensation at the glass edge.

STC - Sound Transmission Class, a rating of how much sound passes through a product. See OITC.

Stile - A vertical member of a sash or door panel.

Stool - The flat interior trim at the sill that extends into the room.

T

Tempered glass - Safety glass that is stronger than standard glass and breaks into small pieces.

Thermal break - A material of low thermal conductivity inserted between high-conductivity members to reduce heat transfer. NFRC states that the break material conductivity is no more than 0.5 W/m.K. See thermal-break aluminum windows.

Threshold - The bottom member of a door frame at the floor. The ADA limits new thresholds to 1/2 in (13 mm) in covered buildings.

Tilt mode - The position of a tilt-turn sash in which the top edge pivots inward for ventilation while the sash stays secured.

Tilt-turn window - A window with one handle and two opening modes, tilt and turn. See tilt and turn windows.

Transom - A window directly above another window or a door.

Triple-pane glazing - Three layers of glass with two gaps in an insulating glass unit.

True divided lite (TDL) - A sash or panel made of several separate smaller panes held in muntins.

Turn mode - The position of a tilt-turn sash in which the whole sash swings into the room on its side hinges. Only this mode counts for egress. See inward-opening windows.

U

U-factor - The rate of non-solar heat transfer through a product. Lower values mean better insulation, and NFRC lists the range as 0.20 to 1.20 for windows. See U-factor for windows.

V

Vent limiter - A device that restricts how far a window opens and can be released only with effort or a tool.

Visible transmittance (VT) - A rating from 0 to 1 for how much daylight passes through a product. Higher means more natural light.

W

Warm-edge spacer - A spacer made with materials of lower conductivity than the metal spacers it replaces, designed to cut heat flow at the glass edge.

Weatherstrip - Compressible material that seals the sash or panel to the frame.

Weep holes - Small holes at the exterior that drain water out of a frame or sill.

WOCD (window opening control device) - A device that limits how far a window opens until it is disengaged for emergency egress, and that re-engages when the window closes.