What Are the Key Parts of a Window?

Large picture window

I bet the last time you looked out your windows was this morning, right?

Windows are magical because they connect the inside of your home with the outside.

But have you ever given your entire window a good stare?

Many of us don’t, which is fine because as long as you can see through the sheet of glass, all is well. Moreover, a well-installed window will serve you for up to 30 years before any replacement windows are needed. It's understandable why we might never pay attention to the different parts of a window.

However, if you are currently shopping for windows, knowing the different parts of a window is essential to make an informed purchase.

See also: A Practical Guide to Window Placement

Common Window Parts

The key parts of a window

Contractors, manufacturers, and window installers seem to speak their own language when discussing windows.

Here are the definitions of parts of the window to help you cut through the jargon and understand the choices you have as a homeowner purchasing new windows.


Within your window framework, the top most horizontal part is called the head. The head mirrors the sill at the bottom of your window frame.


The horizontal structural pieces of the window. These include the lower rail, upper rail, and check rail.


Many windows consist of two sashes, an upper and a lower. Each of these contains its own glass panels, and one or both can slide to open the window. In a single-hung window, the upper sash is stationary, while in the case of a double-hung window, you can lower and raise it for more air circulation. Both upper and lower sashes can have either a single windowpane or multiple panes. These panes are divided by grilles into two or more panels. Some windows have no grilles.


As mentioned above, they are the vertical and horizontal bars dividing your glass into four or more sections. They create the grid pattern you see on the glass. You find them on the inside of:

  • single or double-hung windows
  • bay and bow windows

You can paint them to match your interior design.

In traditional window design, each of these small frames held its own piece of glass. In most modern windows, the grid is just a set of faux frames that overlays a single glass panel.


It’s the horizontal piece of trim sitting across your interior window frame on the bottom.


The vertically mounted interior trim piece located under the stool of your window.

Window Frame

The window frame is the casing that supports and engulfs your entire window framework.

It’s the collective term for the window parts such as the top, the sill at the bottom, and the vertical elements called the jambs. Think of it as the window housing.

Glass Panes

The parts of the window you actually look through are called panes of glass. The glass panes might actually consist of two or three sealed layers of glass for more insulation.

Sash Lock

The sash lock is a mechanism that allows you to lock your window. 


The interior trim pieces on each side of the window.


The lift is the handle you use to raise the lower window sash.


It’s the horizontal part sitting across your window frame on the exterior at the bottom. It mirrors the stool on the inside of your window.

Single- and Double-Hung Windows

These two window types remain the most popular among American homeowners. You can get them in wood, fiberglass, or vinyl materials. A double-hung window comprises two vertical sliding sashes, where you can either open the upper sash or bottom one for air circulation. A single-hung window only one sliding sash: the bottom one.

Picture Windows

Picture windows are the simplest of all windows. They have no moving parts. These inoperable beauties tend to be large, and they are placed facing a pleasing outdoor space, like the backyard. Think of them as picture frames that focus the view.

Slider Windows

This is another simple window design. It features two sashes, and the direction of opening is horizontal. Typically, only one sash slides, and the other is fixed, like a single-hung casement window, but both sashes can be designed to slide.

Casement Windows

A casement window offers a comfortable working mechanism. You open the window panels toward the inside or outside using hinges. Some casement windows open by pushing or pulling, and some have a cranking mechanism.

Casement Window

Exterior Parts of a Casement Window

  • Casement — Do you see where the glass panel sits within the window, the part that you open for fresh air? Yes, it’s what we call the window casement.
  • Stile — These are vertical parts of the casement.

Interior Parts of Casement Windows

  • Mullion — The vertical bar that separates two casements is what you call the mullion in this type of window frame.
  • Transom— On the other hand, the horizontal bar separating two casements goes by the name transom.
  • Fanlight— A small window section above the main window that opens to provide ventilation.
  • Hinges - They attach the sash to the window frame and allow for the window’s opening and closing. They are mainly two or three, depending on window height.

Box Sash Windows

If you live in a very old home, you may have box sash windows. Instead of utilizing springs to make opening and closing easier, they incorporate a weight-and-pully system to balance out the weight of the window and make it easy to slide. A box sash window in good repair operates smoothly, but there are many parts (as you can see below), and it's important to keep them all in working order. Common problems to look out for are accumulated layers of paint and broken sash cords. You may be able to perform simple repairs and maintenance yourself, but for inoperable box sash windows, you may have to call in a repair person who specializes in this type of antique design.

brown windowSource: Pxhere

Exterior Parts of Box Sash Windows

  • Window board— Commonly known as the window nosing in a window frame, they are fitted behind the bricks’ outer part.
  • Top Rail— The horizontal bar across the top of the sash you open.
  • Outer lining - Outer face of the window frame.

Interior Parts of a Box Sash Window

  • Staff Bead— This is the molded bead that attaches to the inside lining holding the two sashes for smooth sliding.
  • Architrave— It is the molding seen on the outermost edge of your window frame from the inside.
  • Sash Cord - You find the cord attached to the sash weight in the box frame. It is fed through a pulley found on the window sides.
  • Sash Bars— This separates the glass with more than one pane.
  • Parting Bead— Vertical seal fitted into a box frame creating a channel for both upper and lower sash.
  • Inside Lining— The inside faces on the head of the window frame.
  • Stiles— This runs vertically on both surfaces of the sash lock.
  • Bottom rail— It is the horizontal bar located over the bottom of the lower panel.
  • Horn— The slight extension at the top stiles near the meeting rails to strengthen the joint.

Window Performance

There is more to a window than its parts. Don't forget to consider energy savings.

U Factor Is How We Measure a Window's Insulative Qualities

Watch out for the NFRC or EnergyStar label to make sure you're getting better quality windows. The U factor is important to make sure you have enough insulation. The lower the number, the less heat your windows will lose. Seattle requires less than 0.27 for the EnergyStar designation. A lower U factor makes for a more comfortable home and saves on your utility bills as well.

Low-E Coating

A low-E coating is an energy-efficient window coating that reflects the heat from the sun in the summer but lets it in when the sun's rays are weaker. In addition to being energy-efficient, low-E windows will reflect the light and allow you to enjoy your views more. They can also help cool your home down by filtering harsh summer light during the heat of the day. Low-e coatings also help maintain a certain level of warmth during the winter months.

Double- and Triple-Pane Windows

Another great option is to choose dual-pane or triple-pane window glass for an added layer of insulation. Even if you have to spend more upfront, the energy savings will be worth it in the long run.

Don't forget to find a reputable contractor to install your windows. A professional window installation is crucial to keeping out noise, minimizing air and water leaks, and maintaining maximum insulation from heat and cold.

See also: Window Replacement 101: The Essential Guide for Beginners

Featured Image: Freeimages

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