What does cill mean?
Cill is a noun used mainly in British English, particularly in architecture and the building trades, where it serves as a variant spelling of sill. It denotes the horizontal member forming the bottom of a window or door frame — the part on which the frame sits and which is typically sloped or weathered to throw rainwater clear of the wall below. The term also extends to engineering contexts, describing horizontal base structures such as the cill of a lock gate or dry dock, against which gates close or vessels rest. Although fully standard in technical British usage and common on construction drawings and specifications, 'cill' carries no meaning distinct from 'sill', and general dictionaries usually treat it simply as an alternative form. Outside Britain, and in most everyday prose, 'sill' remains the expected spelling, making 'cill' a small but genuine marker of professional and regional register.
nounA variant spelling of 'sill': the horizontal member at the bottom of a window or door frame, or the horizontal base of other structures such as a lock gate or a dry dock.
- The horizontal bottom member of a window or door frame, on which the frame rests and which often slopes outward to shed water.
- In civil engineering and building construction more broadly, any horizontal base or foundation course at ground or floor level, as in a lock gate cill or dock cill.
"The builder fitted the new window cill with a slight slope so rainwater would run off it."
"Rain streaked down the glass and pooled on the stone window cill."
"Engineers inspected the lock gate cill for scour damage before reopening the canal."
Regular plural formation, used when referring to multiple window or door bases, most often in technical specifications.
"All the external cills were cast in precast concrete and finished with a drip groove."
It looks like a typo, but 'cill' is an accepted British spelling of 'sill' that builders have been writing on drawings for generations.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cill
The word is a spelling variant of 'sill', which traces back to Old English syll, meaning a foundation or base timber, cognate with related Germanic forms denoting a supporting beam. The 'cill' spelling arose within British architectural and building practice, likely reflecting a perceived connection to Latin-derived forms or simply conventional trade usage, and it became established in technical literature and construction drawings over recent centuries. It shares its Germanic root with 'sill' itself rather than with Latin or Greek vocabulary, so its true cognates are Germanic words for base or foundation timbers.
How cill is actually used
The spelling 'cill' is chiefly British and appears most often in technical and architectural contexts — working drawings, specifications, and trade literature — where it has long coexisted with 'sill'. It is not a separate word with its own meaning, and many modern dictionaries list it only as a variant of 'sill'. Writers outside British building trades should prefer 'sill'.
Easily confused with cill
'Cill' is simply a British variant spelling of 'sill'; both refer to the same structural element, but 'sill' is the standard form in American English and in most dictionaries.
'Ceil' is a verb meaning to line or cover the inside of a roof, whereas 'cill' is a noun naming the horizontal base of a window or door opening.