A guide to common industry terminology for windows and doors that you will find throughout the Yarrow Sash & Door website.

A - C


Active Panel
Primary operating door panel.

A piece of casing or decorative trim installed against the wall immediately beneath the stool of a window.

Argon Gas
A colourless and odourless gas used to fill the airspace between insulating Low E glass. The addition of argon greatly increases the insulating performance of the Low E glass.

A moulding applied to one stile of a double door unit which the other door panel closes against. Used in combination with flush bolts to keep the passive panel closed.

Awning Window Unit
A combination of frame and sash hinged at the top of the vertical jambs which allows the unit to pivot from the top with the sash opening to the exterior of the building.


A system to assist with the positioning of single and double hung windows. Usually located in the jamb liner of double hung or single hung units.

Bay Window
A series of windows installed in a bay which is two flanker units and a center sash; a bay may be an arc or a polygon; when a bay is or closely approaches an arc, the window is termed a bow (see Bow Windows).

The frame member on a double hung window located between the jambs and the casing. The blindstop forms a rabbet that supports either a storm sash or screen.

Bow Window
A series of adjoining window units, installed on a radius.

Brick Mould (Exterior Casing)
An exterior moulding of window and door frames that abuts the exterior facing material of the structure. The casing serves as the boundary moulding for brick or other siding material and also helps to form a rabbet for screens and/or storm sash or a combination door.


Cam Lock
A lever operated lock which is used to prevent intrusion through the sash.

Casement Window
A combination of frame, sash, weather-strip, concealed hinges and operating device assembled as a complete unit. Casements have a flat sill and a sub-sill; screens and/or energy panels are optional. Operating casements have a crank handle for smooth operation.

Metal clad products refer to wood window and door parts which are covered with an extruded permanent coloured aluminum on the exterior of the frame and sash.

Combination Door
A wood framed assembly containing an interchangeable storm panel and screen. The unit is installed on the interior of the door, and is available for wood Yarrow storm doors.

Cottage Window
A double hung or single hung window with unequal height sash.

D - F


Daylight Opening (DLO)
The width and the height of the visible glass.

Depth of the Jamb
The point where the exterior casing or nailing fin ends to the point where the interior casing begins.

Direct Glaze (also known as a Picture Window)
Refers to a window with no sash. The glass is glazed directly into the frame and is stationary.

Divided Lites
Division of light by the use of muntin bars.

  • Muntins: The actual bars that create a pattern in the window.
  • True Divided Lites (TDLs): Glass individually glazed between the muntin bars.
  • Simulated Divided Lites (SDLs): Muntins permanently adhered to the interior and exterior surfaces of the glass.
  • Grilles: Wood Muntins fastened to the interior of the sash to create the effect of divided lites, removable for easy glass cleaning.
  • Shadow Bar: Aluminum bars inserted between SDL muntins to simulate a TDL.
  • Grilles Between Glass (GBG’s): Aluminum flat or contour bars divide the glass visually. Because the grille is between the glass, grilles do not become misplaced or damaged. Cleaning is easy without small panes of individual lites.

Double Hung
Double hung windows have two movable sash which operate vertically. Double hung sash are held in an open position with the use of coil spring block and tackle balancing devices.

Drip Cap
A formed aluminum or vinyl piece which is installed at the top of windows and doors that allows water to run off the casing of the unit instead of seeping around the casing and into the unit.


A decorative plate attached to the stile directly behind the door handle(s). Generally square or rectangular shaped.


A former term used to describe a side or lateral part. Also previously used to describe a 3-wide picture unit or bay. See two-wide entry.
The stationary portion of a window that encloses either the glass (direct glaze) or the sash (operating or stationary) and consists of the head jamb (top), sill (bottom), sub-sill, side jambs, jamb extension, brick mould or flat casing, and blindstop.

Frame Expander
A flat aluminum extrusion used in conjunction with the 90 degree frame expander to provide a flat casing appearance for clad units.

French Casement
A casement styled unit with two sash in one frame providing a sense of openness unrestricted by a vertical mullion or stiles when both sash are open.

G - I


Glass Size (GS)
The measurement of the actual glass, not the visible glass.

Installing glass into windows and doors.

  • Single Glass: Glazing with a single piece of glass.
  • Insulating Glass: two panes of glass separated by a spacer and hermetically sealed together with dead air space between the panes.

Glazing Bead / Stop
Strips of profiled wood or vinyl used to hold the glass in position in the sash. Wood glazing bead is attached to the rails and stiles of the sash using staples, small nails or vinyl barbs. A vinyl bead is held in place by extruded barbs positioned in the kerf. Aluminum caps may be used over the vinyl bead in some cases.

Glazing Tape
A two sided adhesive tape placed between the glass rabbet and the glass and/or the glazing bead and glass of some unit types.

Removable wood dividers made to simulate true divided lites. Grilles are often rectangular or diamond shaped and are easily removed for cleaning purposes.


Handing (also known as Hinging)
A term used to describe the right or left hand operation of a window or door.

Head Jamb
The top member of a frame.


Inactive Panel
Secondary operating door panel.

Inswing Casement
A window with one or two sashes that swing in.

Interior Casing
The casing trim used on the perimeter of the window or door.

J - L


Jamb Extension
A jamb-like member, usually surfaced on four sides, which increases or extends the depth of the exterior or interior window or door frame; jamb extensions imply a larger depth than “wood jamb liners.”


Keyed Cylinder Lock
A cylinder lock that is operated by a key, as opposed to a simple thumb turn, lever or button used to activate the lock. A keyed cylinder lock is a common exterior lockset for a residence and commercial structure.


Laminated Glass
Glass composed of two sheets of glass fused together with a sheet of transparent plastic between the sheets. When broken, laminated glass will generally not leave the opening.

A method of gluing strips of thin clear wood to the lengthwise surfaces of finger jointed material to provide the appearance of clear stock.

A complete door lock system comprised of the lock mechanism together with knobs, keys, plates, strikes and other accessories.

Low-E Glass
Low E stands for low emissivity. The lower the emissivity the higher the percentage of long-wave radiation blocked thereby improving thermal performance. Low E glass is coated with a thin microscopic, virtually invisible, metal or metallic oxide layer. The primary function is to reduce the U-value by suppressing radiative heat flow. A secondary feature is the blocking of short wave radiation to impede heat gain. There are two basic types of Low E glass. The first, vacuum or sputter coated Low E, is referred to as soft-coat (see Low E II definition). The second is pyrolytic Low E, commonly referred to as hard-coat. (See pyrolytic definition.)

Low-E II Glass
A high performance Low E glass, providing the best winter U-value and warmest centre glass. It offers significant improvement in reducing solar heat gain coefficient values, providing customers one of the coolest summer glass temperatures of all Low E products. Additionally, ultraviolet light transmission is greatly reduced. The Low E II coated glass products are specifically designed for insulating glass units normally as a second surface coating. (See Low E and pyrolitic definitions.)

M - O


Masonry Opening
A brick, stone or block opening into which a window or door unit is installed including the outside casing.

Mortise and Tenon
The system by which Yarrow assembles doors and windows. A mortise and tenon joint connects two pieces of wood mainly when the adjoining pieces connect at right angles.

The act of attaching two or more window or door units together. The joint is then finished with a mullion centre cap or mull trim.

The vertical member of a sash, window or door frame between openings in a multiple opening frame.

  • Spread Mull: Two or more units mulled together with a space left between the units. The jamb extension surrounds the entire unit.
  • Stud Pocket: Two or more units mulled together with a space between the units. The jamb extension surrounds each unit separately, providing space for a support member between the units.

Mullion Cover
A clad or wood cover for any mullion post where two windows or doors meet or are joined vertically.

Mullion Reinforcement
The 3/8″ mullion reinforcement is designed to be used on wood or clad multiple casement, awning and direct glaze assemblies to improve structural performance.

Multi-Point Locking System
A line of standard or optional multiple point locking mechanisms installed on the operative panel(s)/ sash of various door and windows to enhance security and performance.

Muntin Bar
A wood section that divides glass into sections. The muntin bars can be either simulated or true divided types.


Nailing Fin
A factory installed vinyl strip that is inserted into a kerf in the frame of clad units. Nailing fin installation is the standard method used for installing clad units.

Non-Keyed Cylinder
A handle without a keyed cylinder. The door cannot be locked from the exterior.


Obscure Glass
Glass formed by running molten glass through special rollers. These rollers have a pattern on them causing the glass to become patterned and thus obscure.

Our drawings always illustrate the window sash or door panels as if you are looking at it from the exterior. X means operating, O means stationary.

An operating sash, panel or unit.

Outside Measurement.

P - R


Jamb Extension
A jamb-like member, usually surfaced on four sides, which increases or extends the depth of the exterior or interior window or door frame; jamb extensions imply a larger depth than “wood jamb liners.”


The resistance a material has to heat flow. Higher numbers indicate greater insulating capabilities.

A groove along or near the edge of a piece of wood.

The cross or horizontal members of the framework of a sash, door or other panel assembly.

Relief Kerf
A kerf (a slot cut) machined into the frame parts of a unit. Relief kerfs inhibit warping and can allow for weather stripping inserts.

A circular cover plate attached to the stile directly behind a knob or door handle. May be plain or have a decorative design embossed into the cover.

A term used to describe the steel drive worm, gears and crank device used for opening awnings and casements.

Rough Opening
The opening in the wall where a window or door unit is to be installed. Openings are larger than the size of the unit to allow room for insulation and to shim the unit square.

Round Top
Generally a semicircle window which is mulled to the top of another window or door, thus forming the round top appearance. There are full round tops, separated round tops, ellipticals, transoms, inverted corners, ovals and Gothic heads, etc. Round tops can be used separately or combined with other units to create a seemingly endless selection.



The operating and/or stationary portion of the window unit that is separate from the frame. The sash consists of the following parts:

  • Stiles: Vertical sash members.
  • Rails: Horizontal sash members.
  • Check Rails: Horizontal sash members that meet, as in double hung units. These could also be vertical check stiles, as in the glider or patio door.

Sash Lock
A locking device which holds a window shut, such as a lock at the check rails of a double hung unit. Larger units utilize two locks.

Sash Opening (SO)
The opening between wood frame members for both height and width (disregarding any jamb hardware tracks).

Sash Width
Horizontal measurement across the face of a sash.

Screen OM (outside measurement)
The width and the height of a screen including wood or metal surrounds.

A close-mesh woven screen material of metal or fibreglass attached to an aluminum or wood surround. Screens inhibit entry of insects, yet permit light, air and vision. Most Yarrow window and door products utilize full screens. Half-screens are available for single hung units.

Side Jamb
The side (vertical) member of a frame.

A stationary glass panel mulled to or installed next to a door. Operating sidelites also available.

The horizontal member forming the bottom of a window or exterior door frame; the lowest member of the frame of a structure, resting on the foundation and supporting the frame.

The extension of the lip of a window sill to the outside edge of the casing.

Single Hung
A window very similar to a double hung window, except that the top sash is stationary or non-operable.

Used to separate the two pieces of glass in an insulating glass panel.

Square Foot
For measuring the area of a unit. RO width (in inches) x RO height (in inches) divided by 144 equals the area in square feet of a unit.

A non-operating sash, panel or unit.

The upright or vertical perimeter pieces of a sash, panel or screen.

A horizontal trim member that laps the window sill above the apron and extends beyond the interior casing. See apron.

Storm Sash
A wood framed assembly containing non-removable glass. The storm sash is removed during the summer and replaced with a wood framed screen.

The supplemental member of a frame used under most awning and casement units as an additional sill with the primary purpose being to hold multiple units together at the sill.

T - Z


Tempered Glass
Float glass panels heated and then cooled rapidly in a controlled environment; this process makes the glass stronger than regular glass. It also makes it safer because when broken it yields small pebble-like fragments.

A pattern of a window unit from which dimensions and measurements can be determined.

Three-Wide (3W)
Current term referring to any product or unit when three frames (i.e. separate jambs) are mulled together as a multiple unit.

A window above a window or door. Transoms can be either stationary or operating.

Turn Button
An aluminum button and screw. Buttons are used to secure wood combinations, storm sash and wood screens to the exterior casing or energy panels to the sash or door panel.


A measure of total heat flow through a window or door barrier from room air to outside air. Lower numbers indicate greater insulating capabilities.

One single product such as a one wide casement.


A strip of resilient material designed to seal the sash and frame members in order to reduce air and water infiltration.

Wire Glass
Glass with wire embedded into the glass when the glass is still in a molten state. This prevents the glass from falling out of the sash if it should break.


The letters OX or XO identify the operation of window or door units as viewed from the exterior. The letter O stands for stationary while the letter X stands for operating.