Welcome to the Progressive Windows Glossary of Terms. This will help to familiarize you with the terminology used both on this site and when you when discussing windows with us. 

Argon Gas
A colorless and odorless gas used to fill the airspace between two pieces of glass in a dual glazed unit. The addition of argon greatly increases the performance of Low-E Glass.

A moulding applied to the stile of a French Door, Sliding French Door or French Casement window unit which the other door panel or window sash strikes. Usually head and footbolt devices will be found on the astragal side.

Single units mulled together

Brick Mould Casing (BMC) 
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.

Check Rails
Horizontal sash members that meet, as in double hung units. These could also be vertical check stiles, as in a horizontal slider or patio door. 

Refers to wood windows and doors which are covered with an extruded permanent colored aluminum jacket on the exterior frame and sash. 

Cottage Window
A window with an unequal sash, top and bottom.

Daylight Opening
The width and height of the visible glass.

Direct Set
Refers to a window with no sash. The glass is glazed directly into the frame and is stationary.

The way by which a person exits. Refers to building codes that require windows to be a certain size in any room a person could use for a sleeping area. 

A measure of a surface’s ability to emit long-wave infrared radiation or room temperature radiant heat energy . Emissivity varies from 0 (no emitted infrared) to 1 (100% emitted infrared). The lower the emissivity, the lower the resultant U-Value

A term used to describe a side or lateral part. Also used to describe a 3-wide picture unit or bay in which single-hungs, double-hungs or casements are attached on either side. 

French Casement
Casement window with two sashes in one frame that opens without being restricted by a vertical mullion when the both sashes are open.

Glass Size 
The measurement of the actual glass, not the visible glass

Installing Glass into a window

Removable wood divider made to simulate true divided lites. 

Term used to describe the right or left hand operation of a window or door.

Insulating glass
Another term for Dual Glazed

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.

Laminated Glass
Glass composed of two sheets of glass fused together with a sheet of transparent plastic between the sheets. 

Low-E Glass
Low-E stands for low emissivity. Low-E glass coated with a thin, virtually invisible, metal or metallic oxide layer. The idea is to reduce the amount of ultraviolet and solar heat gain allowed to come into or escape from your home. 

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

The act of attaching two or more window or door units together.

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

Muntins, or ‘Munt’
Horizontal or vertical ‘bars,’ extending from a bar to a stile or rail or another bar. Used in reference to divided lites or grilles. 

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 for installing windows.

Obscure glass
Glass formed by running molten glass through special rollers. This gives the glass a texture making it translucent. 

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

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

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

Also called retro-fit, refers to units which are sized for replacement over an existing window frame.

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.

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

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

A wedge or strip of wood used to level and square a window or door in the rough opening

A stationary glass panel mulled to or installed next to a door.

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

Spacer or Spacer Bar
Used to separate the two pieces of glass in a Dual Glazed unit. This may be just around the edges of the glass, in a pattern if Modern Divided Lites are used. 

A semi-elliptical area, the lower center is the point where the divided lites meet and extend outward in spokes.

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

Similar to the starburst, this pattern also contains one elliptical bar that divides the window into 2 arches through which the remaining bars spoke.

Tempered Glass
Tempered glass is a both stronger and safer than regular glass, as it is tougher to break. If it is broken, it breaks into little beads rather than shards. Tempered glass may be required in certain areas (i.e. within 24′ of a door)

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