Cill - sill
The second element in the architectural term windowsill can confuse. It has been spelled sill since Old English, because of its Germanic roots (there are cognates in German, Dutch and the North Germanic languages). Since the eighteenth century, it has from time to time been spelled cill, with the same pronunciation, rhyming with 'till' and 'bill': IPA: /sɪl/.
- In ordinary use, sill (or, as AWE does not recommend, cill) is most commonly associated with 'window', denoting the horizontal flat item at the bottom of a window opening, usually of timber if indoors, and stone or concrete if in the external wall.
- Originally sill denoted a strong timber as the foundation of a wall, particularly that of a timber-framed house. By extension, sill was used for horizontal members, usually at the bottom, of other frames and constructions. (In wooden ships, the port-sills were fitted at top and bottom of the portholes.)
- Various analogous meanings exist, such as 'door-sill', for the threshold of a door, and ridges in the sea-bed which similarly make a boundary over which currents of water have to 'step'. There have been various uses in geology, of which the most relevant may be "In modern use, a tabular igneous intrusion lying parallel to the surrounding strata" (OED, 1910).