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Basic Terminology

Here are basic terms used on vinyl windows on alphabetical order.

A / B / C / D / E / F / G / H / I / J / K / L / M / N / O / Q / R / S / T / U / V / W / Y / Z


ACCESSORY GROOVE: A serrated groove extruded into the window frame that is designed to receive a number of accessory profiles typically used to complete or aid installation of the window.

AWNING WINDOW: A window consisting of several top-hinged sections arranged in a vertical series, operated by one or more control devices that swing the bottom edges of the sections outward, and designed especially to admit air while excluding rain.


BALANCE SHOE: Part attached to bottom of balance that attaches balance to sash. A bar or pin on the sash fits in a “slot” in the shoe.

BALANCE SPRING: A device for counterbalancing a sliding vertical sash, generally associated with a double-hung or single-hung window, so that it can be held open at a position. See Sash Balance.

BAY WINDOW: (Not available with HK Windows) 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.

BEAD: A sealant after application in a joint irrespective of the method of application, such as sealant bead, glazing bead, etc. Per ASTM E 631, n – in glazing, (1) a strip of metal, wood or plastic used around the periphery of a pane of glass to secure it in place. (2) a strip of sealant, glazing compounds or putty.

BEVELED: A sloping surface or edge.

BLIND STOP: A sash or window frame member applied to the exterior vertical edge of the side and head jamb in order to serve as a stop for the top sash and to form with the brick moulding and/or casing a rabbet for the storm sash, screens, blinds and shutters.

BOTTOM RAIL: The bottom horizontal member of a window sash.

BOW WINDOW: (Not available with HK Windows) Composed of three or more individual windows in a gently curved contour. Bow windows also project from the wall of the structure.

BRICK MOULDING: An exterior (milled) trim piece to cover the gap between the window frame and masonry in a masonry opening or in other siding.


CAP / CAPPING: Cosmetic covering, usually found on the exterior of the window or door to achieve aesthetic sight lines or to integrate the window or door system into the building surface or weatherproofing system. If panning is being used for weatherability, the panning is not considered cosmetic, but part of the window system.

CASEMENT WINDOW: A projecting window with a single sash hinged at the sides and usually opening outward like a door and operated by a (crank) handle, which turns to open or close the unit.

CASING (Trim): Exposed molding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall.

CAULK: To seal cracks and joints around window and door frames to prevent leakage of water and air.

CAULKING: A mastic compound for filling joints and sealing cracks to prevent leakage of water and air; commonly made of silicone-, bituminous-, acrylic- or rubber-based material.

CIRCLE TOP WINDOW: A window having a curved (radius) top and a flat bottom. The shape of the window is an exact half-circle with the height being usually one-half of the width. Also called circle head, half-circle and half-round.

COTTAGE-STYLE WINDOW: Hung Window (Single or Double) that has a larger bottom sash (lite) than the top sash (lite).


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-STRENGTH GLASS: Sheet glass with a thickness between 0.115" to 0.133" (3 to 3.38 mm) or approximately 1/8" used in larger window units. This is HK Windows standard glass size.

DOUBLE WINDOW: Two windows separated by a mullion, forming a unit. Also called a twin window unit. HK Windows use term "Twin Window".


EGRESS: -v To “go out”. n. A means of exiting. An egress window is one, which is large enough for an adult to exit the room in case of an emergency. The size will be defined by national or local building codes.

EXTENSION JAMBS: Flat parts made of vinyl, wood or other materials which are attached to the inside edges of a window jamb to extend it in width to adapt to a thicker wall.

EXTRUSION: Compacting and melting a plastic material and forcing it through an orifice in a continuous fashion. In the extrusion process, the material is conveyed through the heated machine barrel by a helical screw where it is heated and mixed to a homogeneous state and then forced through a die of the shape required for the finished product.

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. In older homes, low, inward-opening windows with a bottom-hinged sash. These attic windows built into the top molding of the house are sometimes called 'lie-on-your-stomach' windows or slave windows. Often found on Greek Revival and Italianate houses.


FENESTRATION PRODUCT: Any transparent or translucent material plus any sash, frame, mullions, dividers, in the envelope of a building, including but not limited to: windows, sliding glass doors, french doors, skylights, curtain walls, and garden windows.

FIXED LIGHT (Fixed Sash): Window section which is non-operative (does not open). I.E. Top IG/Sash of a Single Hung unit, which only bottom sash is operable sliding vertically.

FIXED UNIT: A non-operable stationary unit.

FLASHING: Sheet material that protects and bridges the joint between the window or door frame members and the adjacent construction for the purpose of preventing water penetration by draining water away from the window or door to the exterior. See also Through-wall flashing.

FRAME: Outside member of a window (or door) unit which encloses the sash.

FUSION WELDING: The process of heating mitered corners to over 200°F and bringing the heated corners into contact until they fuse together into a single piece of vinyl.


GLASS: A transparent, translucent or opaque material formed by fusing silicates with soda or potash, lime and sometimes various metallic oxides.

GLAZING: The glass panes or “lites” (lights) in the sash of a window. Also the act of installing the glass in a window sash.

GLAZING (Double): A single glazed sash with an additional glass panel installed on the sash to provide an air space between the two lites of glass. The second glass can either be removable (RDG) or fixed and can be installed on either the inside or outside of the sash. Double glazing differs from insulating glass in that there is no positive seal around the edges of the two lites of glass to provide a true dead air space and there's no desiccant within the unit to absorb and hold moisture.

GLAZING BEAD: A removable trim that holds the glass in place in a window sash.

GLAZING BLOCK: A small, hard rubber block placed around the edges of the glass unit in a window sash to position the glass and prevent it from shifting.

GLAZING COMPOUND: A pliable substance applied between the window sash and the lites of glass to seal against the elements and sometimes to adhere the glass to the sash.

GLAZING TAPE: Double-sided tape used to adhere glass to sash and form an airtight, watertight seal.

GLIDING WINDOW: Same construction as a sliding window. The moving sash generally travels on rollers. HK Windows, Sliding Windows

GRILLES or GRID: Ornamental or simulated muntins and bars which don't actually divide the lites of glass. Generally made of vinyl or wood and fit on the inside of the sash against the glass surface for easy removal. Grilles or grids between the glass are usually made of aluminum and sealed inside the insulating glass unit.

GBG: Grid(Grilles) Between the Glass


HALF CIRCLE OR HALF ROUND: A half moon shaped stationary unit. Arch is half of the width of the unit.

HALF ROUND WITH LEGS: A half moon shaped stationary unit with extended legs. Arch is half of the width of the unit.

HALF SCREEN: A screen, which does not cover the entire opening of a window. Used on the bottom half of single hung units and on the operating sash side of single sliders.

HEAD: The top or upper member of any elementary structure; in windows, it refers to the top of the frame, as in Round Head Window, head jamb or header.

HEAD EXPANDER: An inverted U-channel installation accessory that may be fitted to the head of a replacement window to accommodate differences between rough opening and window heights.

HEAD JAMB: Also called “header” or “head”: Cross or horizontal jamb member forming the top of the frame.

HINGE: A movable joint enabling a window to swing open.

HORIZONTAL GRID: Horizontal grid bar.

HUNG WINDOW: Window with one or more hanging (counter balanced) sashes.


INFILTRATION: Leakage of outdoor air into a house, such as through cracks around sash or window frame.

INSTALLATION ACCESSORIES: Components supplied by the fenestration manufacturer that are specifically designed to mate or “trim out” the product with various surrounding constructions.

INSTALLATION FIN OR FLANGE: A vinyl or metal flange inserted into or an integral part of the side and head jambs of a window unit used for installing the window in an opening.

INSTALLATION HOLES: Holes in window or doorframes that are fabricated by the manufacturer to locate and accommodate installation fasteners.

INSULATING GLASS OR IG UNIT:Double glazing with an enclosed, dehydrated, and hermetically sealed air space between the panes; the space is commonly from 3/8" to 5/8".

INSULATED WINDOW: A window with multiple glazing that provides one or more air spaces between layers of glazing.


JAMB: A vertical member at the side of the window frame; also refers to the horizontal member at the top of the window frame, as in Head Jamb and Window Jamb.

J-Channel: A manufacturing component of vinyl or aluminum siding systems which have a curved channel that the planks fit into, used around windows and doors to make a weathertight seal.


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.


LAMINATED GLASS: Two or more sheets of glass bonded together with a plastic inner layer to produce a stronger, more break-resistant glass. May also have wire mesh or other reinforcing embedded in the glass sandwich for extra strength.

LATCH (Catch/Lock): A device which holds a window shut, such as the latch at the meeting of a double-hung window or one mounted on the stile of casement windows, often referred to as Lock.

LIFT RAIL: Handle for raising the lower sash in a double-hung or single-hung window.

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. This is typically spelled “Lite” to differentiate from sun light or other sources of light that shine through a window.

LOCK: A fastening device in which a bolt is secured and can be operated by a key. Commonly used to refer to Latches or Catches.

LOCK RAIL: Rail on a door located at the proper height to receive the lock and, for that purpose, usually made broader than the other rails. See Meeting Rail.

LOCK STILE: Stile of a door to which the lock is applied, as distinguished from the hinge stile.LOUVERED WINDOW: A window having louvers or slats that fill all or part of the opening. See Jalousie Window.

LOW-E GLASS: Stands for Low-Emissivity. Glass that has been given a special micro-thin coating (usually silver & metal oxides) 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).


MASONRY OPENING: The opening in a masonry wall to accept a window or door unit, the same as a rough opening in a frame wall.

MEETING RAIL (Lock Rail): One of the two horizontal members of a double-hung sash, which come together.

MEETING STILE: The vertical members in a pair of sash, as in a horizontal sliding window.

MULLION: Vertical or horizontal divisions or joints between single windows in multiple units. Can be either decorative or functional (structural). Decorative only versions are also mull casings or covers.


NAILING FIN: An extrusion that attaches to the window frame and used to secure the unit to the rough opening.

NIGHT LATCH: Latch mechanism on the inside of the top sash that retains the window in a partially open position for ventilation.


OBSCURE GLASS: A glass (frosted, etched, fluted, ground, etc.) for privacy, light diffusion, or decorative purposes.

OPERABLE WINDOW: Window, which can be opened for ventilation.

OPERATOR: Crank-operated device for opening and closing casement or awning windows.

ORIEL WINDOW: A window projecting from the wall and carried on brackets, corbels or a cantilever. Similar in appearance to a Bay Window. The term is also applied to a style of window with a larger top sash than its bottom sash. It is the opposite of a cottage-style window.


PANE: A sheet of glass for glazing a window. After installation, the pane is referred to as a “lite” (light) or 'window lite'.

PICTURE WINDOW: Large fixed windows; introduced in the 1940's.

PIVOT PIN (Pivot Bar): Part mounted on or in the end of the sash that fits into balance shoe and from which the sash may be tilted or pivoted in.

PLASTIC The word "Plastic" means capable of being shaped or molded to some definite form or shape during the manufacturing process and possessing the property of permanently retaining the shape into which it was molded or extruded.

PRAIRIE GRID: A prairie style window is divided into 6 or 9 unequal panes. A prairie window has four small square corner panes (on 9Lite Prairie) per IG or small square corner panes on 2 top corner of top sash and 2 bottom corner of bottom sash (on 6Lite Prairie), four long rectangle edge panes, and one large center square pane.

PULL:A handle for opening a window.

PVC (Polyvinyl chloride): An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows.


QUARTER ROUND WINDOW: Stationary or operating window with glass shaped as a quarter circle; it is often divided into separate panes by a removable grille, installed on the interior or grille between the glass.


RAIL: Horizontal members of a window sash or door panel.

REPLACEMENT WINDOW: A window that is designed for and subsequently installed after removal of all or part of a previously installed window.

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. The R-value of an ordinary single-pane sash with a 15 mph wind on one side is about 0.9.

ROUGH OPENING: The opening left in a frame wall to receive a window or door unit. The jack studs on each side, which supports the header across the top, form it. Cripples support the rough sill at the bottom. The rough opening generally allows 1/2" or more in each dimension in excess of the window or door unit dimension. Openings in brick walls are known as masonry openings.


SAFETY GLASS: Annealed glass that undergoes further processing becomes a safety glass. The characteristic of safety glass is to reduce the possibility of severe injury upon accidental impact. There are two types of safety glass that meet the (CPSC-federal standard) 16 CFR 1201, Cat. II: Tempered and Laminated. Tempered glass, through a heat strengthening process, becomes four times stronger than annealed glass and when broken, separates into dice-like cubes approximately the thickness of the glass. Laminated glass is two lites of glass sandwiched together with an interlayer of polyvinyl butyral (PVB) under heat and pressure. Laminated glass when broken tends to remain intact. Usually referred as Tempered Glass

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. Usually Glazed, meaning with and IG unit installed.

OPEN SASH: A sash in which the glass has not been installed.

SASH AND FRAME: A window and its cased framing.

SASH BALANCE: A device for counter-balancing a sash of a double-hung or single-hung window to hold it in the up position. There are four basic types:

SPRING: A balance using a spring for counter-balancing; introduced in the 1980's;

SASH LIFT: A handle for raising the lower sash.

SASH LOCK: Generally, a cam-action type lock applied to the rails of a window or at the open edges of a projecting window to pull the rails tightly together or to seal the sash tightly to the frame, both for security and weather tightness.

SASH STOP: A molding that covers the joint between window sash and the jamb.

SHIMS: Wood wedges (often wood shingles) used to position the window unit in the rough or masonry opening in a square, level and plumb position during (and after) installation.

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.

SILL ANGLE: An L-shaped installation accessory that may be employed at the sill of a replacement window to accommodate the slope of the existing sill construction.

SINGLE-HUNG WINDOW: A window that is similar to a double-hung window except that the top lite (light) is fixed (stationary).

SINGLE SLIDER A sliding window. Usually XO or OX, X being side operable outside looking in. I.E. XO = Left Side operable outside looking in.

SPACER: The material used around the edges of an insulated glass unit to separate the panes of glass. They may be hollow metal tubes of aluminum or steel, or non-conductive materials such as silicone, butyl, etc. or a combination of materials.

STATIONARY SASH: A fixed sash; also referred to as deadlite sash.

STUDS: Vertical wood framing members which form a frame wall. In typical construction, these are 2 X 4''s about 8' long.


TEMPERED GLASS: Special heat-treated, high-strength safety glass which shatters into pebble-sized particles but not into long slivers, when broken.

TILT PIN: A metal or hard plastic nail-like pin fastened to the sash to allow it to engage the balance shoe and from which the sash may tilt or pivot in. See Pivot Pin.

TINTED GLASS: Glass that has been given a slight shading or coloring (tint) to it, usually to control sun light in very bright, high-intensity sun locations. There are many colors available today such as bronze, gray, green, etc.

TRANSOM: Generally refers to an opening or stationary sash above a door or window which serves a similar purpose to a side lite. A transom joint is the horizontal joining area between two window units, which are stacked one on top of the other.


U-VALUE: A value indicating the rate of heat flow through a building construction (combination of materials), expressed in units of 'Btu/h per square foot of surface per degree F. difference between indoor and outdoor air temperature.' This is numerically equal to the 'inverse of the sum of R-values' for the construction. See Heat Transfer Coefficient.

UNITED INCHES: (U/I) The sum in inches of the width and height of a window unit. Common “call size” for replacement windows.


VERTICAL GRID: Vertical grid bar.


WARM EDGE SPACER: Use of a non-conductive edge spacer in insulating glass units instead of the conventional metal (conductive) edge spacer. “Warm Edge” spacers may be made of butyl, silicone foam or other non-metallic materials and sealants.

WELDING (FUSION): Joining thermoplastic pieces by one of several heat-softening processes. A properly welded corner is as strong, or stronger, than the extrusion itself.

WINDOW: A glazed opening in an external wall; an entire unit consisting of a frame, sash and glazing and any operable elements.

WINDOW FRAME: The fixed frame of a window, which holds the sash as well as the operating hardware for the window.

WINDOW SCHEDULE: A listing of windows required in a given house, stating types, sizes, number of lites (lights), manufacturer, and any special needs, etc.

WINDOW SCREEN: Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.