Parts of a Window: The Terms on Your Quote and Invoice

Window quotes and invoices use trade shorthand: "replace IGU," "new stool and apron," "Z-bar frame," "mulled unit." None of it is complicated, but it is hard to compare two quotes when you are not sure what the words point to. This page is the reference. It is part of our California window guide, and the shorter A-to-Z list for both siding and windows lives in the siding and window glossary.

The two big pieces: frame and sash

Every window is two assemblies. The frame is the fixed outer structure fastened to the house. The sash is the part that holds the glass. On an operable window the sash moves inside the frame; on a picture window it does not move, and on some fixed windows the glass is set directly into the frame with no separate sash at all (called direct glazing).

That distinction matters on a quote because the two can be replaced independently. A "sash replacement" or "glass replacement" keeps your frame. A "retrofit" or "insert" window puts a new frame and sash inside your old frame. A "full-frame" or "new-construction" replacement takes the old frame out down to the rough opening. Those are three different jobs with three different amounts of wall disturbance, and our page on retrofit windows covers when each one makes sense.

Every part, in one table

The frame, piece by piece

The frame has four sides and three names. The top is the head, the two sides are the jambs, and the bottom is the sill. Manufacturers are not perfectly consistent here: some glossaries call the top and both sides "jambs" and say "head jamb" for the top. If a quote says "jamb," it means a frame side.

The sill is the one that fails. It is the lowest point, it takes the water, and on older wood windows it is where rot starts. A properly made sill slopes toward the outside so water runs off rather than sitting.

Sill, stool and apron are three different things

This is the most common mix-up on a quote. The sill is part of the window frame. The stool is the flat interior trim piece that sits on top of it and projects into the room, the thing most people call the windowsill. The apron is the flat trim on the wall directly under the stool.

Many California tract houses built since the 1970s have no wood stool at all. The drywall wraps into the opening on three or four sides (a "drywall return") and the bottom ledge is drywall or a slab of cultured marble. On those houses a retrofit window usually leaves the interior untouched.

Casing and brickmould

Casing is any trim that frames the window and covers the gap between the unit and the wall. Interior casing is the picture-frame trim inside. Exterior casing does the same job outside, and brickmould is a specific thick, profiled exterior casing, traditional on wood windows, that the siding butts into. Stucco houses mostly have neither on the outside: the stucco runs to the window frame or to a foam-and-stucco trim band. Our exterior window trim guide covers what gets replaced and what gets re-used.

The sash and the glass

The sash is built from stiles (vertical) and rails (horizontal). Where two sashes meet in the middle of a hung window, the two overlapping rails are the check rail or meeting rail. On a horizontal slider the same joint runs vertically.

The IGU and its spacer

Modern windows do not hold single sheets of glass. They hold an insulating glass unit: two or three panes separated by a spacer at the edge and sealed all the way around. The space between the panes holds air or a gas fill. Low-E coatings sit on one or more of the inner glass surfaces.

The spacer is the least visible part and one of the more important. It is the only solid path between the inner and outer pane, so what it is made of affects how cold the glass edge gets in winter. "Warm-edge spacer" on a spec sheet means a spacer designed to conduct less heat than a plain aluminum one.

When the edge seal fails, humid air gets in and the window fogs between the panes where you cannot wipe it. That is an IGU failure, not a frame failure, and on many windows the unit can be swapped. Our page on why windows fog between the panes explains when that repair is worth doing. A separate guide, glass-only replacement versus a new window, goes through the decision.

Glazing bead

The glazing bead (some manufacturers say glazing stop) is the strip around the perimeter of the glass that holds the IGU in the sash and covers its edge. On vinyl windows it is a snap-in vinyl strip. On older wood windows the same job was done by glazing putty. To replace an IGU, a glazier pops the beads, lifts the old unit out, sets the new one on setting blocks and snaps the beads back.

Mullion versus muntin

They sound alike and they are constantly swapped, including by salespeople.

• A mullion is the joint between two separate window units combined in one opening. A three-part window with a fixed center and an operable unit on each side has two mullions. Units joined this way are "mulled."
• A muntin is a bar that divides the glass within one sash. Historically each small pane was a separate piece of glass and the muntins held them. Today most are grilles applied to, or sealed inside, a single IGU.

The practical difference: a mullion is structural and a wide mulled assembly needs to be designed for the opening. A muntin is a design option. The construction types (between the glass, simulated divided lite, true divided lite) are compared in our guide to window grilles.

The parts you do not see

Balances

A single-hung or double-hung sash is heavy, and something has to hold it where you leave it. That is the balance: a spring or block-and-tackle mechanism concealed in the jamb. Old wood windows used a cast-iron weight on a cord in a pocket beside the jamb. If a sash slams shut or will not stay up, the balance has failed. It is a repairable part on most windows, provided the manufacturer still supplies it.

Weep holes

Weep holes are the small slots or holes on the outside face of the sill, easiest to find on sliding windows and sliding doors. The sill track of a slider is designed to collect water and drain it out through the weeps. Water standing in the track during a storm is normal. Water that never drains is not, and the usual cause is weeps blocked by dirt, paint or caulk. Never seal them.

Nailing fin

The nailing fin is a thin flange around the outside of a new-construction window frame. It is nailed or screwed to the wall framing, and the wall's water-resistive barrier and flashing tapes lap over it. Once the stucco or siding is on, you cannot see it.

This is why the fin defines the installation method. To use a nail-fin window in an existing wall you have to expose the framing around the opening, which on a stucco house means cutting back and patching stucco. A retrofit window avoids that. In California the common retrofit frame has an exterior flange that covers the old frame from outside, sold as a flush-fin or Z-bar frame, while a block frame has no fin at all and sits inside the opening. The three are compared in nail fin, flush fin and block frame windows.

Performance terms that sit next to the part names

A quote will also carry numbers. They are not parts, but they describe the parts above:

• U-factor: how readily the whole window, frame and glass together, lets heat through. Lower is better.
• SHGC (solar heat gain coefficient): how much of the sun's heat comes through. Lower blocks more.
• VT (visible transmittance): how much daylight comes through. Higher is brighter.
• Air leakage: how much air passes through the assembled window. Lower is tighter.

All of them are printed on the NFRC label, and our guide to reading an NFRC label explains each one. In California the first two are what the Energy Code checks, by climate zone; see what Title 24 requires of your new windows.

Reading a quote with this in hand

Most of the real differences between two window quotes come down to five lines. Find them on each quote and compare like with like:

1. Frame material and product line. Named, not "premium vinyl."
2. Installation type. Retrofit into the existing frame, or full-frame to the rough opening. This changes how much of the list above is new.
3. Glass package. Number of panes, Low-E type, gas fill, spacer, and the NFRC U-factor and SHGC for that configuration.
4. Trim scope. What happens to interior casing, stool and apron, and to exterior trim or stucco.
5. What is being repaired around the opening. Sill rot, flashing, damaged framing. A quote that is silent here has not looked, or is leaving it for a change order.

Our guide on how to compare window replacement quotes goes line by line.

Sources

• Andersen Windows: Window and Door Glossary
• Marvin: Glossary of Window, Door and Marvin Product Terms
• National Fenestration Rating Council: The Energy Performance Label
• NFRC Consumer Guide to Windows (Efficient Windows Collaborative): Window Frames

Working with Heritage Exteriors

Our quotes name the frame, the installation type, the glass package and the trim scope in plain terms, and we will stand at the window with you and point to each part if that is what it takes to make the quote readable. If a competing quote uses a term you cannot place, bring it along. Licensed in California, CSLB #1055919. Call (916) 546-2991.