English is full of fascinating quirks. Words that look like they should rhyme—such as come and home or through and cough—don’t. Other words, like tear and tear or wound and wound, are spelled the same but can have completely different pronunciations depending on their meaning. Then there are expressions like “raining cats and dogs” and “going cold turkey,” which sound perfectly normal to native speakers but can leave language learners confused.

One everyday word that rarely gets a second thought is o’clock. We use it constantly when telling time, but have you ever wondered what the “O’” actually means—or why there’s an apostrophe there at all? It isn’t simply decorative. The expression developed from a much older way of telling time, when clocks weren’t the only way people determined the hour. The missing words have survived quietly inside “o’clock” for centuries. Continue below to discover what the “O’” originally meant.