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Actual Dimension – The outside horizontal and vertical measurements of a window or door excluding the nailing fins. Also referred to as overall dimension.

Advanced Low-E Insulating Glass with Argon – Pella’s insulated glass system designed for many U.S. climates. Unique Low-E technology delivers balanced insulation for cold winters and hot summers.

AdvancedComfort Low-E Insulating Glass with Argon – Pella’s insulated glass system designed for northern U.S. climates. Unique Low-E technology delivers balanced insulation for colder winters and moderate summers.

AdvantagePlus® Protection System – Pella’s low-maintenance protection system for hinged patio doors and fiberglass and steel entry doors that feature Pella’s patented rot-resistant jamb-on-sill design, an EnduraClad® exterior frame and dual-seal weatherstripping.

Air Infiltration – The amount of air leaking in and out of a building through cracks in walls, windows, and doors.

Air Leakage (AL) – Measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf·m/ft².

Airspace – The measured distance between the inner surfaces of the two panes of glass in an insulated unit. Also used when referring to the thickness of the spacer bar between two panes of glass.

Aluminum-Clad Window – Durable, low-maintenance aluminum clad exteriors that help protect all-wood windows and doors. See also Cladding.

Annealed Glass – Non-tempered glass. The most common glass used in window products; breaks into shards.

Argon Gas – A colorless, odorless, non-flammable, non-reactive, inert gas sealed between two panes of glass, which acts as an insulator.

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

Awning Window – A single-sash window that opens outward to the exterior from the bottom. Awnings are typically wider than they are tall.


Balance – See Sash Balance.

Basement Window (Hopper Window) – A sash that is hinged at either the top or the bottom and swings in to the interior (Pella Vinyl product).

Bay Window – A composite of three windows usually made up of a large center fixed unit and two flanking units which are typically angled 30 or 45 degrees to the wall. A bay window refers to the angle of departure from the plane of a wall.

Between-the-Glass Blinds or Shades – Blinds or fabric shades tucked between panes of glass of a window or door — protected from dust, damage and little hands. See also “Snap-in between-the-glass blinds or shades.”

Bottom-Up Shades or Blinds – Shades or blinds that can be raised from the bottom to any position.

Bow Window – A composite of four or more window units in a radial formation. Typically at a 12- or 14-degree angle to the wall.

Brickmould – Exterior casing around a wood window or door through which nails are driven to install the unit. Covers the gap between the frame and masonry opening. In some cases, siding is installed up to the edge of the brickmould.

Bumper – A soft vinyl or rubber cushion that prevents the moving panel of a sliding glass door from striking the jamb on the fixed panel side of the door.

Butt Hinge – A standard barrel-type hinge, mounted by mortising the leaves into the sash and frame of a window.

Butyl Tape – See Sealant.


Cam-Action Lock – A pivoting latch usually attached by a screw or rivet to the top rail of a single- or double-hung sash or the centermost side rail of a sliding window sash. The latch locks to the meeting rail, some type of keeper, or strike plate attached to the meeting rail.

Caming – The metal channels that separate and secure individual pieces of decorative glass together to create the overall decorative glass design.

Casement – A window unit in which the single sash is either fixed or opens outward to exterior on the left or right side. Casements are generally taller than they are wide.

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.

Caulking – A mastic compound for filling joints and sealing cracks to prevent air and water leaks; commonly made of silicone, bituminous, acrylic, or rubber-based materials.

Checkrail – Bottom rail of the upper sash and/or the top rail of the lower sash of a single- or double-hung window. The lock is typically mounted on the checkrail. Also known as meeting rail and lock rail.

Cladding – An aluminum material bonded to the exterior wood frame on Pella's wood windows and patio doors.

Clavos – Decorative pieces of metal that resemble the large tacks used to join wooden door planks together.

Combination (Joining) Mullion – Factory-assembled combinations that are created by mulling together various combinations of fixed or venting windows and doors.

Composite – A window or door consisting of two or more sashes in one frame utilizing an integral mullion.

Condensation – A deposit of water vapor from the air on a cold surface that is below dew point temperature.

Contemporary Double-Hung Window – Double-hung window where the upper sash is taller than the lower sash. Typically a 60% (top sash) to 40% (lower sash) ratio.

Contemporary Sliding Patio Door – Patio door with narrow door panel stiles and rails. Provides more visible glass area.

Corner Key / Corner Lock – A metal or plastic device used to secure the corners of a sash, frame spacer bar or screen section. The device is generally an L-shaped part that fits inside mitered ends of the perimeter parts. The corner key can be crimped (staked) in place or screws can be inserted. The corner key makes a rigid joint possible.

Cottage Double-Hung Window – Double-hung window where the upper sash is shorter than the lower sash. Typically a 1/3 (top sash) to 2/3 (lower sash) ratio.


Decorative Glass – Variety of glass types and textures assembled into decorative patterns using metal caming.

Desiccant – A drying agent, such as silica gel, used by some manufacturers between the panes of insulating glass to help prevent fogging between the panes.

Design Pressure – The pressure a product is designed to withstand. This value is a measure of a product’s capacity to withstand the forces of wind loading in both positive and negative directions, while it is closed and locked.

Direct Set – A window in which the glass is stopped directly to the frame, without utilizing a sash.

Divided Lights – See Simulated-Divided-Lights and Integral Lights Technology.

Double-Hung Window – Window with two sashes that move vertically – one can be raised from the bottom and the other lowered from the top for two levels of ventilation.

Double-Pane Insulating Glass – Two panes of glass with a hermetically sealed air space between the panes of glass. This space may or may not be filled with an inert gas, such as argon.

Drip Cap – A horizontal molding that diverts water from the top of the window or door unit so that the water drips beyond the outside of the frame.


Egress – A code that requires a minimum opening of a window for persons to exit or firefighters to enter a home or building. The dimension of the net clear opening of a window or door (the horizontal clear distance, vertical clear distance and the net clear opening) are established by local building codes.

Elliptical – A door or window having a top rail with an egg-shaped radius ending in a rounded point at its apex. A fixed window having a curved top and a flat bottom. The shape of the window is a half-ellipse; often used over another window or door opening.

EnduraGuard® Wood Protection – Pella’s exclusive wood protection formula contains a combination of three active ingredients for strong protection against the effects of moisture. This advanced formula is collectively stronger than a single active ingredient, providing a broader base of protection against the effects of moisture, decay, and stains from mold and mildew. Plus, an additional ingredient adds protection against termite damage.

ENERGY STAR® – ENERGY STAR certified products, such as windows and doors, mean these items use less energy, save money and help protect the environment. Behind each blue label is a product, building or home that is independently certified to use less energy and cause fewer of the emissions that contribute to climate change. Today, ENERGY STAR is the most widely recognized symbol for energy efficiency in the world, helping families and businesses save $300 billion on utility bills, while reducing greenhouse gas emissions by two billion metric tons since 1992. For more information, go to

Extension Jamb – A board used to increase the depth of the jambs of a window frame to fit a wall of a given thickness.

Extrusion – A form produced by forcing material through a die. Pella wood window frames use extruded aluminum cladding for added protection. Pella fiberglass products use extruded Duracast® fiberglass composite. Pella vinyl products use a vinyl extrusion.


Fenestration – An architectural term referring to the arrangement of windows and doors in a wall; one of the important elements in controlling the exterior appearance, ventilation, light and energy efficiency of a home or building. From the Latin word, “fenestra”, meaning window.

Flashing – A thin strip of metal or synthetic material that diverts water away from a window, door or skylight.

Foam Insulation – Optional foam insulation inserted into the frame of Pella® 350 Series and Pella® 250 Series products to increase energy performance.

Foam Spacer – Non-glare foam spacers placed between insulating glass in line with simulated grilles permanently bonded to the inside and outside surface of the glass to give the appearance of individual panes of glass. Combines the look of true divided light with the performance of insulating glass. See also Integral Light Technology®.

Frame – The enclosure in which window sashes or door panels are mounted.

French-Style Sliding Patio Door – A sliding patio door with wider panel stiles and rails, giving it the look of French hinged doors.


Glazing – The process of applying or installing glass into a window sash or door panel. Also refers to the type of glass used in the process.

Glazing Bead – A vinyl or wood strip applied to the window sash around the exterior perimeter of the glass.

Glazing Stop – The part of the sash or door panel, which holds the glass in place.

Grilles – Wood, vinyl or metal bars that visually divide window or door glass, giving the appearance of multiple panes of glass. Available in different widths, profiles and patterns. Available as removable or permanent grilles depending on the brand of window or door. (Also known as grids or muntin bars)

Grilles-Between-the-Glass – Grilles permanently installed between the panes of insulating glass.


Head – The main horizontal member forming the top of the window or door frame.


ILT – See Integral Light Technology®.

Impact-Resistant Glass – An advanced polymer layer sandwiched between two layers of glass – provides exceptional strength. The glass is able to withstand an impact equivalent to that of a 2-by-4 traveling at 50 feet per second. Also dramatically reduces outside noise and harmful UV rays and offers added protection from intruders and forced entry.

Insulating Glass – A combination of two or more panes of glass with a hermetically sealed air space between the panes of glass. This space may or may not be filled with an inert gas, such as argon.

Integral Light Technology® (ILT) – Grilles are permanently bonded to the inside and outside of energy-efficient insulating glass with a non-glare foam spacer between the panes to give the illusion of individual panes of glass. Provides the look of true divided light with the performance of insulating glass.

Integral Mullion – A horizontal or vertical section which is bounded at either end or both ends by a crossing frame section.

Integrated Crank With Fold-Away Handle – The fold-away crank on Pella's casement and awning windows improves operation and aesthetics – and doesn’t interfere with roomside window treatments.

Interlocker – An extruded piece of aluminum that is attached between a fixed panel and vented panel of a door to provide stability, security and added weatherproofing.

InView™ Screens – Pella’s high-transparency InView screens are clearer than conventional fiberglass screens. They allow 14% more light and 8% more air to increase airflow. Improved airflow is based on calculated screen cloth openness. Screen cloth transmittance was measured using an integrated sphere spectrophotometer.


Jamb – The vertical sections that form the sides of a window or door frame.

Jamb Depth – Dimension from the back of the fin or brickmould to the interior frame edge of a window or door frame.

Jambliner – In modern double-hung windows, the jambliner is installed inside the jambs where the window sashes slide up and down.


Knocked Down (KD) – Unassembled, pre-manufacturered window or door parts intended for assembly at a later date on the job site.


Laminated Glass – Pella's laminated insulated glass features an advanced polymer layer sandwiched between two layers of glass to reduce outside noise and harmful UV rays, and add protection against intruders and forced entry.

Low-E Insulating Glass – Low-emissivity (Low-E) glass coating adds insulation and helps reduce heat transfer through the glass, keeping more heat out in the summer and more heat in during the winter. Plus, it helps block most of the UV rays that can fade window treatments, carpets, walls and furniture.


Masonry Opening – The space in a masonry wall left open for the window or door.

Metal Spacer – Simulated-Divided-Light Grilles with the option of a metal spacer between the panes of glass give the illusion of true divided light with the performance of insulating glass.

Mortise – A slot or rectangular cavity cut into a piece of wood to receive a tenon.

Mortise-And-Tenon – A strong wood joint made by fitting together a mortise in one part and a matching projecting tenon in the other. Used to construct the corner joint in Pella's wood sashes.

Multipoint Lock System – Pella's locking system that secures the door at multiple points for added security.

Muntin Bar – Bar that visually divides window or door glass. Also known as a grille. Also known as a window pane divider.


Nailing Fin – A vinyl or aluminum extension attached to the frame of a window or door which creates a positive seal between the window and the frames wall. Acts as an additional barrier against air and water leakage. Screws or nails are fastened through the fin to hold the unit in the opening.

NaturalSun Low-E Insulating Glass with Argon – Pella's insulated glass system designed for very cold climates like far northern U.S. Allows the sun's heat to flow in and warm the home, while providing excellent insulation from the cold.

NFRC (National Fenestration Rating Council) – A nonprofit organization that empowers consumers who are in the market for energy efficient windows, doors and skylights. When you see the NFRC label on these products, this is your assurance that it's going to perform the way it's advertised to perform. See more at

Non-Decorative Glass – Glass with no embellishments; clear glass.


Obscure Glass – Glass with a textured surface that dramatically reduces visibility while allowing natural light through the glass.

Outdoor-Indoor Transmission Class (OITC) – Standard used for indicating the rate of transmission of sound between outdoor and indoor spaces in a structure.


Palladian Window – Large, arch-top window flanked by smaller windows on each side.

Parting Stop – Narrow strip, either integral or applied, that holds a sash or panel in position in a window or door frame.

Patio Door (French Hinged Door) – A door composed of either single hinged door, two movable doors that swing open from the center, or one movable with one fixed door.

Patio Door (Sliding Door) – A sliding door compromising of one fixed panel fitted and one operating panel that slides horizontally to provide ventilation and passage. Available in two-, three- and four-wide units.

PerformaSeal® – Pella's patented PerformaSeal design creates a weathertight seal that helps block out the damaging effects of harsh weather. Plus, it provides a lower sill to make coming and going easier.


R-Value (Thermal Resistance) – A measure of resistance to heat flow of a 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.

Rail – The top and bottom horizontal sections of the framework of a window sash.

Reinforcing Mullion – A horizontal or vertical connecting unit with an added continuous mullion stiffener that is used to join two or more individual windows or doors.

Removable Grilles – Interior roomside grilles that can easily be removed when you need to clean the glass.

Renovation® Double-Hung Window – Fully assembled replacement double-hung window that slides into the existing sash "pocket" of an old window. Designed for pre-1950 wood double-hung window frames (units with balance cords, pulleys and hidden weights). Trim, paint, wallpaper, plaster and siding are usually not disturbed. For use in replacement applications only.

Rolscreen® Retractable Screen – Pella's self-storing screen rolls away and out of sight when not in use. Available on select Pella® casement windows and sliding patio doors.

Rough Opening – The opening left in a wall that will receive a window or door unit. The rough opening usually allows 1/2 inch or more in each dimension in excess of the window or door unit dimension.


Sash – A single assembly of stiles and rails that holds the glass in the frame.

Sash Balance – A device that holds the vertically sliding sashes of double- or single-hung windows in any desired position through the use of weight to counterbalance the weight of the sash.

Sash Lift – Handle used to assist in raising and lowering the sash of a double- or single-hung window.

Self-Closing Screen Door – Screen door that closes gently after someone enters or exits.

Sill – The horizontal sections that form the bottom of the frame of a window or door.

Simulated-Divided-Light Grilles – Grilles permanently bonded to the interior and exterior of the insulating glass, simulating the look of divided light. Available with or without a metal spacer between the panes of glass.

Single-Hung – The look of a double-hung window with the upper sash fixed and the bottom sash that can be vertically raised from the bottom for ventilation.

Sliding Window – Windows with one fixed panel and another panel that slides horizontally. (Also known as horizontal slider)

SmartFlash® Tape – Pella’s SmartFlash installation foil backed butyl flashing tape is a part of the Pella Window and Door Installation Method.

Solar Heat Gain Coefficient (SHGC) – Measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits.

Sound Transmission Class (STC) Rating – A rating of how well a product attenuates airborne sound. A noise reduction of 10 decibels represents cutting the noise level in half, as interpreted by the human ear. So a rating of 25 means that the product reduces the outside noise by approximately 25 decibels, cutting the noise in half 2-1/2 times, or cutting it by over 80 percent.

Stile – The vertical sections of the framework of a sash.

Stool – A shelf-like board of the interior part of the window sill where the bottom rail of the sash closes.

SunDefense™ Low-E Insulating Glass with Argon – Pella’s insulated glass system designed for very hot, sunny climates. Allows in visible light and provides a clear view, while helping to block the heat of the sun.

SureLock® System – Pella’s patented unison locking system on casement and awning windows, which uses one handle to secure the window at two locking points. Locks reach out to lift and pull the sash against the weatherstripping to form a tight seal.


Tenon – A rectangular projection cut out of a piece of wood for insertion into a mortise.

Triple-Pane Glass – Three panes of glass with a hermetically sealed air space between the panes of glass. This space may or may not be filled with an inert gas, such as argon.


U-Factor – Measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft2·°F. The lower the U-factor, the better the window insulates.


Value Proposition – Pella's marketing strategy highlighting six key areas: Brand, Product, Order Fulfillment, Sales Support, Profit Opportunity and Customer Satisfaction.

Visible Glass – The outside horizontal and vertical dimensions of a glass lite (pane), excluding the sash or cladding.

Visible Transmittance (VT) – Measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see.


Weatherstrip – A strip of resilient material that covers the joint between the window sash and frame in order to reduce air leaks and prevent water from entering.

Weep Holes – Small holes drilled along the bottom exterior edge of a vinyl window or door to allow any water to drain out and away from the unit.