Cills

Cills are the bottom horizontal members of window openings, and in some dialects the spelling cill is used instead of sill. The plural form cills refers to more than one such element. In modern usage the term is often interchangeable with sill or windowsill, but cill is more commonly found in historical or regional texts, particularly in Scotland and parts of northern England.

Exterior cills are part of the window assembly and may project beyond the wall to shed rainwater and protect the wall surface. Interior cills lie inside the opening and provide a finished surface at the bottom of the window frame. Materials vary widely: stone and carved stone cills are common on older buildings, while modern cills may be timber, metal, concrete, brick, or PVC, with exterior variants sometimes featuring a drip edge or slight overhang to improve drainage.

Construction and maintenance considerations focus on moisture control and structural integrity. Proper flashing and weatherproofing are essential to prevent water ingress at the joint with the wall. Timber cills require regular maintenance, such as treatment, sealing, and repainting to resist rot. Stone or masonry cills can weather and crack and may need repointing or repair of any decorative profiles. When replacing cills on historic properties, matching the original profile, material, and scale is often recommended to preserve architectural character.

Etymology and usage notes: the spelling cill is chiefly found in British English and Scottish sources, while sill is the more common form in American English.