It's common to see windows trimmed like picture frames -- with thin casing and no stool. Unfortunately, that treatment ignores the window stool's architectural function. Built with an apron and proportional casing, a correctly sized stool grounds the window. The stool also divides a room at the proper height, which helps establish the underlying sense of correctness found in a well-proportioned interior.
--Brent Hull is a historic millwork consultant. Reach him at www.hullhistorical.com.