People say it every day without thinking ― "two o'clock," "six o'clock," "eight o'clock sharp." But what is the purpose of that little "o" and apostrophe? Is it short for something? Why do we only use it for certain times?
(If you're a fan of "The Gilded Age" on HBO, you might even wonder ― does it have anything to do with the fan favorite character affectionately called "Clock Twink"?)
The origin of "o'clock," however, goes back much further than the Gilded Age. It doesn't stand for "hour," and it's not derived from some whimsical Irish surname, either.
"O'clock" is a shortened form of the phrases "of the clock" and "of clock," which people began using to communicate the time after the invention of clocks. Saying it was five "of the clock" essentially meant five "according to the clock."
"When mechanical clocks became more widespread in Europe around the 14th century, people needed a way to distinguish the hour shown on a clock from other ways of telling time, such as by the sun or church bells," said Esteban Touma, a cultural and linguistic expert at the language-learning app Babbel. "If someone said 'three,' that could once have meant three hours after sunrise or simply a rough point in the day. Saying 'three of the clock' clarified that you meant the specific hour indicated on a mechanical clock face."
The phrase "of the clock" appeared in writing as early as 1384 (as "ten of the clokke"), with "of clock" showing up in 1419 ("eyghte of clok"). There's recorded evidence of the shortened "o'clock" ― written as "four oclock" ― dating as early as 1560.
"The contracted form, 'o'clock,' became standardized in Early Modern English, around the 16th and 17th centuries, and has remained in everyday use ever since," Touma said, describing the term as "a fossil ― a surviving fragment of a much longer medieval phrase that we no longer use in full."
But how exactly did that shortened version come to be? According to English professor Michael Adams of Indiana University, it's easy to understand from a phonetic standpoint.
"With 'of,' that consonant is at the front of the mouth and 'clock' is at the back of the mouth," he explained. "And we try as hard as we can not to move freely between the front and back with our consonants. As a result, we decide we want to have a contraction instead of the full form 'of clock,' which dies out pretty quickly a couple of centuries later."
In other words, moving quickly from the "v" sound in "of" to the hard "cl" in "clock" requires your mouth to shift position in a way that English speakers prefer to avoid if possible.
Over time, some people also began writing and saying "a clock" in addition to "o'clock." Pronounced like "uh clock," this is an example of the soft "schwa" vowel sound ― which is the most common sound in the English language. Basically, the tongue tends to save energy and stay in the middle of the mouth for syllables that aren't stressed.
"When there's a vowel in an unstressed syllable in English, it tends to go to the center of the mouth ― what we call the vowel space ― and it produces the 'schwa,'" Adams said. "In the 16th century, we've got evidence of people saying 'a clock,' as opposed to 'of clock' or 'o'clock.'"
Still, "o'clock" prevailed as the preferred term for expressing the time over "of the clock" or "of clock."
"This kind of phonetic erosion is very common in English," Touma said. "We see it in words like 'goodbye,' which comes from 'God be with ye.'"
Another example with the 'o' contraction is jack-o'-lantern, which is a shortening of "Jack of the lantern."
Touma noted that o'clock is also a "rare example" of a contraction that only persists in one very specific grammatical context.
"We only use 'o'clock' with full hours ― 'three o'clock,' 'six o'clock' ― which preserves the structure of the original phrase referring to the numbered hour on the clock face. That specificity makes it a small but fascinating window into how English evolves."
Indeed, we don't say "four thirty o'clock" or "six fifteen o'clock." In recent years, however, people have gotten creative.
"It is our tendency to modify, develop and play with the language we've got," Adams said. He pointed to expressions like "martini o'clock" and "it's five o'clock somewhere" as particularly transgressive and playful innovations ― ways people use language to justify (or joke about) their drinking habits.
"It's a good reminder that the way we speak shifts with culture, technology and real life," said Madeline Enos, a language trends expert and communications manager at Preply. "What feels new today can become second nature tomorrow. That's the beauty of language. It grows with us, and we grow with it."
The rise of "o'clock" demonstrates a broader vocabulary shift that accompanied a major technological change.
"The rise of mechanical clocks in the 14th century created a need for more precise timekeeping vocabulary," Touma said. "'O'clock' is a linguistic trace of that moment ― it marks the point when time stopped being measured primarily by nature and started being measured by machines."
Touma finds "o'clock" to be an interesting example of how telling time varies across languages as well.
"In French and Italian, people often express time relative to the next hour ― for example, 2:45 is 'trois heures moins le quart' in French and 'le tre meno un quarto' in Italian so 'three minus a quarter,'" he said.
Meanwhile, people say 3:30 in German as 'halb vier' ― literally "half four" to express that it is halfway to four.
"This is a quirky contrast to English's straightforward 'three thirty' or 'half past three,'" Touma said. "It's a reminder that even something as universal as telling time can differ a lot linguistically and culturally."