StateLine Exteriors | Kansas City

The Anatomy of a Window

When you’re trying to replace a window, you’ll likely encounter a range of new terms you don’t know. Most people know a window has a sill and glass, and that’s about it. But the expert window installers at StateLine Exteriors can help you understand all of the parts of a window and help you make better decisions when you’re choosing new ones. We want to empower you with knowledge, not overwhelm you with high-pressure sales tactics. To that end, we’ve created this brief guide on the anatomy of a window.


The window frame is what keeps the window from just sitting against your drywall. The frame and the various parts of it are stationary, and they support the window. Windows are installed into frames, but new windows and window replacements might require your window professional to adjust or fix the frame.

You may also want to be familiar with the various parts of the frame:

  • Head: The main horizontal bar at the top of the frame.
  • Jambs: The vertical parts of the frame, they are on either side of the frame.
  • Sill: The bottom of the frame.  
  • Jambliner: A strip for the sides of the window frame (the jambs) which allows the sash to fit in properly. 


The window sash is a part of the window proper. It is the portion of the window that moves. So, on a double-hung window, the sash is the panel that moves up and down. Some windows do not open, and they do not have a sash. Other windows open in more ways than one and may have two moveable panels or two sashes.

The sash contains rails. Including:

  • Upper rail: The top horizontal bar on the sash, normally made of aluminum, vinyl or wood.
  • Lower rail: The bottom horizontal bar on the sash. It is in direct contact with the glass (just like the upper rail), and it is made of the same material.
  • Check rail: On double-hung windows specifically, this is the rail that contacts the other sash.
  • Lift: This is a handle attached to the sash, which allows you to lift it. Not all windows have lifts.

Fixed Panel

If your window has a portion that does not move, that part is called the fixed panel. Typically, this is the top portion of a double-hung window. It might also be the pane of glass used in a picture window.

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The glass is, of course, the glass that makes up the center of the window and allows you to see through it. While once windows just had a single piece of plain glass across them, window construction has gotten safer, more energy-efficient, and more complicated. There is now a lot to know about the glass of your window.

The type of glass used is one of these things. Glass on windows should be at least tempered. This means that the glass is treated in a way that when it breaks, it does not shatter into sharp and dangerous shards. Instead, it should crumble into tiny pieces. Another form of glass, called laminated glass, remains in place when broken.

There are also other kinds of glass you may find for windows, including tinted glass, mirrored glass, and Low-E glass. These are normally reserved for unique situations for commercial uses and are rarely used in homes.

Glazing refers to the number of layers of glass in the window’s construction. In between these layers of glass, we put colorless gases. Most windows will have argon, but some will use krypton. We use these gases because they are very dense. They serve to block heat, without blocking your view. Essentially, they boost the insulative property of your window, keeping heat in your home during the winter and heat out of your home during the summer. Without glazing and these gases, windows would hugely undermine the energy-efficiency of your home.


Casing is an optional element of the window. It is a decorative form of framing which makes the window look better. Minimal designs may not have much casing, but traditional windows or those in older styles may have a significant casing.


If your window cranks open, then it has an operator. This is the little hand-powered crank that can open or close the window sash.


Weatherstripping is an energy-conscious improvement for windows. It is a strip of rubber-like material which covers up the space between the window sash and the frame. It is meant to insulate the window better and prevent the transfer of air and heat.


When you have multiple window sections, the mullion is the part that holds them together. A mullion may run vertically or horizontally. A window may have more than one mullion.


Have you ever wondered how they get several little panes to stick together to form one decorative window? This is not several glass panes but one large one divided up by grilles. These are small, usually grey or black, lines that divide up the pane for aesthetic effect.


If your window opens, then it may include a screen. These are woven meshes, usually plastic or fiberglass but sometimes also metal, which are meant to keep out bugs and outdoor debris without blocking air.

Weep Hole

Any “weep” element of a building component is meant to allow water to pass through. The weep holes on window sills allow any condensation that has built up to pass and prevent water damage to the window.


Muntins are an alternative to grilles. They are decorative frames which add a grid over the top of the glass. They were originally about holding the glass in place but may also be decorative.


There are sometimes boards under the windowsill, also called aprons. They are horizontal and decorative. StateLine Exteriors can help you choose the perfect window and then install it for you. Contact us for our window installation service or to ask your questions about quality windows.

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