Abstract: The subject of this patent application, the Sill Saw, is an application-specific handheld circular power saw that affords the replacement of narrow frame windows, usually aluminum frame windows, with windows having thicker (deeper) frames, without the removal of the window casing materials or associated inside window moldings. This handheld power tool is specifically designed for the purpose of sawing window casing boards to a narrower width without removing them from the structure. There is no other method of accomplishing this, short of disassembling the window components, installing the new window, modifying the removed components and reinstalling them. Current, general purpose, handheld power saws cannot cut the window casing boards completely to the corners; that is, to their intersections with one another. This can be accomplished using the subject of this patent application, the Sill Saw.