What is a Stool?

Definition of Stool in Construction
A piece of trim on a window unit that extends the sill of the window and projects out into the interior of the structure beyond the plane of the interior trim. Normally an apron would be installed under the stool. The term stool is a basic construction, descriptive term. Almost any type of projection that extends beyond the standard plane of the surrounding surface can be called a stool. Normally the term applies to a horizontal piece of material that projects out on a 90 degree angle from the surface plane.