I am in the process of planning my first ever window replacement and am trying to understand the anatomy of a window, inside and out. Here are two photos (inside and outside) with some labels, and I am trying to understand what all of these labeled parts are called, and what purposes they serve:

Inside: enter image description here

Outside: enter image description here

So we have our table of labeled parts:

Part        What it's called        What it does
A           Top Plate (?)           Gives the top part of the window something to nail to (?)
B           Side Plate (?)          Gives the side of the window something to nail to (?)
C           ???
D           ???
E           ???
F           ???
G           ???
H           ???
I           ???
J           ???
K           ???
L           ???
M           ???

Could someone please provide some course correction and steering here and let me know what all these parts are called and what it is that they do? What style of window is this? It is critical that I know this stuff for when I go to research the actual replacement (which is next). Thanks in advance!

1 Answer 1


This is either a single- or double-hung window, depending on how many of the sashes are operable.

A just looks like the wall framing header. There's often a two-by laid flat along the bottom and members on edge above, but not always.

B is the original window jamb (frame), or an extension. Sometimes window and door jambs aren't deep enough for the walls they're in.

C is the lower jamb, or sill. It's sloped, which is why the side jambs (B and F) extend below at the inside.

D is the head jamb.

E and F appear to be stops, which are what retain the sashes. Sashes are the moveable (operable) or stationary frames that contain the glass.

G looks like a trimmer (or jack) stud, but it's hard to see. They're studs that support the header. Ordinarily they don't stop at the framing sill, but things vary.

H might be a support block. Not sure about that or I. Neither are likely important in your case.

J is the framing sill. It's a horizontal member to which new window flanges might attach, and it supports siding, drywall, and trim.

K and L are brick mold, or casing. It covers the gap between framing (sheathing) and the jamb.

M is the outside edge of the sill mentioned earlier.

  • Thanks so much @isherwood (+1) -- a few followup questions if you don't mind. (1) you say that E and F retain the sashes...where are these "sashes" in either of my photos? (2) It sounds like, as you work from the external-most edges of the Windows, to the inner-most (window itself), you're dealing with framing which consists of headers, sills and studs, then you're dealing with jambs which affix to the framing, and then finally you either have the Window or the Window plus stops which affix to the jambs, is this correct? Commented Sep 27, 2022 at 14:09
  • (3) For J you mention window flanges, where are the flanges in either of my photos? And finally (4) for K and L you imply that brick molding and casing are interchangeable, is that true? Thanks again so much! Commented Sep 27, 2022 at 14:10
  • "... new window flanges..." Your old windows don't have them. It's a modern thing.
    – isherwood
    Commented Sep 27, 2022 at 14:17
  • Brick mold is just a style of casing. It was traditionally used against protruding brick.
    – isherwood
    Commented Sep 27, 2022 at 14:18
  • 1
    A "trimmer stud" is also referred to as a "jack stud" or "cripple stud". "Glass" is also referred to as "lites" or "panes" in a divided-light or multi-pane window (sometimes they are simulated using one piece of glass with muntins on each face instead of between the panes). A couple of additional terms: mullion, glazing (plus others). See Wikipedia for a good reference. Commented Sep 27, 2022 at 22:17

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