Learning Center > Term Glossary

Term Glossary



Inside flat trim member, which is used under the stool at the bottom of the window.


The center member of a double door, which is attached to the inactive door panel.

Awning window

A window that has a single sash in which the bottom of the sash swings out.


Bay Window

Three or more windows in a row, usually made up of a large unit in the center and one unit on either side at 30° or 45° angles to the wall.

Bow Window

Four or more window units together in a bow formation.


Outside moulding around the exterior window frame designed to seal gaps between the window and the opening.



Inside casing is a flat, decorative moulding that covers the inside edge of the jambs and the rough openings between the window unit and the wall.

Check rail

On a double-hung window, the bottom rail of the upper sash and the upper rail of the lower sash, where the lock is mounted.

Comfort Factor

Refers to in home thermal comfort specifically, the impact of window glass on room temperature.



A space, which protrudes from the roof, usually including one or more windows.

Double rafter

The doubling (side by side) of the roof members to reinforce an opening in the roof for a slope-glazing installation.


Use of two panes of glass in a window to increase energy efficiency and provide other performance benefits.


A window with two vertical operating sashes that slide up and down.

Drip cap

A moulding placed on the top of the head brick mould or casing of a window frame.


Egress window

A window large enough for exit or entry in case of an emergency.



An architectural term referring to the placement of windows in a wall.


A way of joining pieces of wood together to form longer lengths.


Non-venting or doesn’t open.


A metal or plastic strip attached to the outside of the head or side jambs to provide a weather barrier. It prevents leakage between the frame and the wall.



A strip of material used to create a watertight seal between the sash and the frame of a window.


The glass panes or lights in the sash of a window.

Glazing bead

A plastic or wood strip applied to the window sash around the glass.

Glazing stop

The part of the sash or door panel which holds the glass in place.


A bar used to visually separate glass into multiple lights. Also called a window pane divider or a muntin.



The main horizontal member forming the top of the window or doorframe.

Head Board

A flat board that fits the contour of a bow or bay window at the top and that finishes off the ceiling inside the window.


A heavy beam extended across the top of the rough opening to help support the weight of the building.


Jamb Liner

Metal or plastic covering the inside surface and head jambs of sliding windows.



The part of a casement window lock that looks like a hook.



Handle or grip installed on the bottom rail of the lower sash of a double-hung window to make it easier to raise or lower the sash.


(also spelled lite) Glazing framed by muntins and/or sash in a window or door.


A common term used to refer to glass, which has low emissivity due to a metal coating on the glass surface between the two lights of glass to restrict the passage of radiant heat.


Masonry Openings

The opening in a masonry wall to accept a window or door unit.


A slit cut into a piece of wood to receive another part.


The joints between single windows in a multiple window unit.

Mullion Casing

An interior or exterior casing to cover the mullion joint between single windows.


A bar used to visually separate glass into multiple lights. Also called a window pane divider or a grill.



A metal arm and gear, which allows for easy operation or closing of projecting windows.

Outer Frame Member

The exterior protruding portion of a window frame, which has no exterior casing.



Usually refers to the separate panel or panels in a doorframe.

Picture Frame Casing

The use of casing on all four sides of the interior of a window, replacing the stool and apron at the sill.


The degree of the slope of a roof.



Structural members of a roof that support the roof load.


The horizontal members of a window sash or door panel.

Rough Opening

The opening left in a frame wall to receive a window or door unit.

Rough Sill

The horizontal rough framing area which forms the bottom of the rough opening.


Sash Balance

A system of weights, cords and/or coiled springs which assist in raising double-hung sash and tend to keep the sash in any placed position by counterbalancing the weight of the sash.

Sash Cord

In double-hung windows, the rope or chain that attaches the sash to the counter balance.

Sash Lock

A lock applied to the checkrails of a sliding window or at the open edges of a projecting window to pull the checkrails tightly together or to seal the sash tightly to the frame, both for security and weather tightness.

Seat Board

A board that fits the contour of a bow or bay window at the bottom and that provides a seat or shelf space.


Wooden wedges that are used to secure windows in the rough opening. Helps make the window level within the rough opening.

Side Lights

Tall, narrow, fixed or operating sash on either or both sides of a door to light an entryway or vestibule.


Horizontal member that forms the bottom of a window frame.

Sill Course

The row of bricks across the bottom of a rough opening.

Simulated Divided Light

A method of constructing windows in which grills are attached to the inside and outside of a panel of glass to simulate the look of true divided light.

Single Glazing

Use of single panes of glass in a window.


A double-hung type of window in which the top sash is fixed or inoperable.

Solar Gain

The readings of how much heat is collected inside a home through the glass of windows and doors.


The vertical side member of a window sash or door panel.


Inside horizontal trim member of a window sash or door panel.


A wood trim member nailed to the window frame to secure window parts in place.


Wood framing members, used vertically, to build a frame wall. In construction, these are eight-foot-long 2” x 4”s.



A smaller window above a door or another window.

Triple Glazing

A sash glazed with three lights of glass, enclosing two separate air spaces.



A measure of heat transmission through a wall or window. The lower the U-Factor, the better the insulating value.


Vapor Barrier

A watertight material used to prevent the passage of moisture into or through floors, walls and ceilings.

Venting Unit

A window or door unit that opens or operates.



Force exerted on a surface by moving air.

Window Pane Divider

A short bar used to separate glass in a sash into multiple lights. Also called a muntin or a grill.