If you’ve ever traveled through Europe or stayed at an international hotel, you may have noticed the letters “WC” on a restroom door. For many people, the meaning is not immediately clear because the abbreviation is rarely used in everyday American English. WC stands for “water closet,” an older term for a small room or space containing a toilet.

The word “closet” once meant more than a storage space. It could also describe a small, private room. A “water closet” referred to a toilet that used water to flush waste away. As indoor plumbing became more common during the 19th century, the expression became widely associated with indoor toilet facilities and was eventually shortened to WC.

The history behind the term is connected to the development of flushing toilets. In the late 16th century, Sir John Harington described an early flushing toilet, while in 1775, Scottish inventor Alexander Cumming patented an improved design. His toilet included an S-shaped water trap, which helped stop unpleasant sewer gases from entering buildings, a basic principle that remains important in modern plumbing.

WC became especially useful on signs because it was short, easy to recognize, and suitable for international locations. Today the abbreviation can still be found in hotels, airports, train stations, and public buildings across Europe and other parts of the world, while in the United States people usually say “bathroom” or “restroom” and encounter WC mainly when traveling abroad.