A sill is the flat, horizontal piece that forms the bottom of a window frame or doorway. If you've ever sat on a windowsill reading a book or watching the street below, you've used this sturdy ledge that runs along the base of the window. The sill helps keep rain from seeping into the wall and gives you a perfect spot to display plants or favorite objects.

In construction, sills play an important structural role. A door sill (also called a threshold) is the piece you step over when entering a building. Builders also use the word for the bottom horizontal piece of a wall frame that sits directly on the foundation.

You might hear the word in the phrase “from sill to rafter,” meaning from the very bottom to the very top of a building. Geologists use the word differently: a sill in geology is a flat layer of hardened volcanic rock that pushed its way between existing rock layers underground, like a thick sheet of stone sandwiched in the earth.