The trim around a window, known as casing, makes the physical and visual transition between the window frame, or jamb, and the surrounding wall area. Stylistically, casing may take any one of dozens of forms, ranging from Colonial to Craftsman to Greek revival to Modern and everywhere between, with variations limited only by the imagination. But regardless of the style, the approach to casing the window is similar.

There are two basic ways of casing windows: the “picture-frame” method, in which the casing detail is the same on all four sides of the window, and the more traditional style with a stool (commonly called a windowsill) and apron. In this article, we’ll focus on the traditional approach. However, because the top of the casing gets mitered, we’ll touch on the important aspects of the picture-framing method, as well.

As a finish trim, window casing is meant to be seen, so it’s important to select good millwork stock from a reputable supplier. Using top-quality stock will help ensure that the contours of the profile line up properly at miters, that the casing lies flat against the wall, and that all the casing joints stay tight.

When handling any type of molding material, including casing stock, take care not to damage the finished faces. As a rule, I always avoid having millwork or trim delivered in wet weather, and once it arrives at a job, I always make sure that it’s stored properly on site in clean, dry, and stable conditions.

To explain the procedure for trimming windows

Prerequisite skills:

  • Ability to take accurate measurements
  • Familiarity and basic skills with sliding compound miter saw
  • Familiarity and basic skills with table saw
  • Familiarity and basic skills with pneumatic finish nailers
  • Familiarity and basic skills with routers
  • Familiarity and basic skills with biscuit joinery


  • Basic hand tools including measuring tape, dividers, and combination square
  • Compound miter saw
  • Table saw
  • Pneumatic or cordless finish nailer
  • Router with small diameter round-over bit
  • Biscuit joiner (optional, but recommended)
  • Clam Clamps (optional, but recommended)

Extension jamb: Lengths of wood that build out a window frame flush with the interior wall surface.
Casing: Material that finishes the joint between the extension jambs and the interior walls.
Stool: The horizontal finish material extending from the lower part of the window frame that is a termination point for the casing and the extension jambs.
Apron: Horizontal finish material below the stool.
Reveal: Small offset between layers of material that creates a visual step or line in the finish.
Miter: An angled cut across the flat part of a profile that usually forms a 90° joint.
Bevel: An angled cut perpendicular to the profile that usually forms a 90° joint as the return.
Return: A small triangular piece of a profile that completes a profile back to the wall.
Biscuit joinery: A method for joining two pieces of wood using a slot and pre-formed oval biscuit.

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