The interior sill of a window is known in professional terms as a stool. The stool is a piece of a wood with a rebated bottom, which typically features a 45-degree angle. This rebate, at the bottom face of the stool, allows the stool to be seated on top of the window sill and then nailed into place with finish or brad nails. The stool is a decorative part of a window and is installed in a home or building to create a traditional-style look. Removing the sill will require you to pull the bottom apron trim, along with the two vertical casing trims, out from the window for ease of access.
Search for the horizontal trim, also known as the apron, at the bottom of the window just below the stool. Separate any paint or caulk sealer from the apron by scribing along the outer edge with a utility knife.
Slide the flat end of a small pry bar along the bottom edge of the apron and pry it out from the wall. Pull finish nails out from the apron and wall, using the hammer's claw, and set them aside to be reused.
Search for the two vertical pieces of trim, also known as casings, on the left and right sides of the window and scribe along the outer edge with the utility knife to separate any paint or caulk sealer.
Remove the side casings from the left and right side of the window by sliding the flat end of the small pry bar under the outside edges and pulling it out. Pull the finish nails from the casing or side jamb out of the window.
Slide the flat end of the small pry bar between the bottom face of the stool and either the rough or finish sill, depending on how the window was fabricated. Pry the stool up, using the pry bar so that the nails are pulled out of the sill. Slide the stool out.
Be careful not to pry too hard when removing the casing, apron or stool, as delicate window trim can crack easily when under stress. The inside window trim is made up of five pieces. Working down the window, the horizontal piece of trim at the top of the window is the upper casing. The two pieces of trim along the two vertical edges of the window are referred to as side casings. The lower horizontal piece of trim below the sill is called the apron. Just above the apron is the stool.