Why do we use "o" in telling the time, e.g. 9 o'clock?

I looked it up in the dictionary but it did not say anything about the letter.


Used to specify the hour when telling the time.

  • ‘the gates will open at eight o'clock’

What does the "o" stand for?

  • 4
    In this case, the o' has a historical derivation, but speakers don't think of it as short for anything anymore. You can't replace it with of the. So this is a matter of etymology; it's not like "lots o' luck" where o' represents a reduced colloquial form of of. You'll have to learn o'clock as a single word. – user230 Apr 12 '14 at 16:08

This website explains it quite nicely:

o'clock (adj.) c.1720, abbreviation of of the clock (1640s), from Middle English of the clokke (late 14c.).

The usage of o' as a shortened way to say of is not uncommon (this is called an apocopic form, when the last syllable or consonant is left unpronounced). Many o' these are relatively rare in written form, but you might see them when an author was trying to capture colloquial elocution in a quote:

  • "All I needed was the queen o' hearts for a straight flush!"
  • "Yer startin' your new job tomorrah, Jim? Well, best o' luck to ya!"

For some reason, o'clock is one exception where the o' became not only common, but formal.

"colloquial elocution" is a fancy way of saying "how people say words informally"

  • or "how folk really talk" – CoolHandLouis Apr 12 '14 at 13:35

The accepted answer here correctly explains that the "o'" contraction is short for "of the", but it does not explain how it came to be in the first place.

Prior to the introduction of clocks, there were other indicators of time, like celestial bodies. Even when clocks were introduced, very few people had them, and common people still used other means to tell the time. (It should be noted that the concept of counting time in hours precedes clocks). It was customary to refer to e.g. the fourth hour of the day based on the time elapsed since sunrise.

However, with the introduction of clocks, it became necessary to distinguish time told from one means to the time told by a clock, as they would not always correspond. (The sun comes up earlier in summer and later in winter). Therefore, to make the distinction that time referred to was being told by a clock, one would say "of the clock".


8 o' clock is correctly said as 8 on clock

  • Can you say where that is the case? It does not correspond to what I hear in the UK. – mdewey Mar 22 at 16:49

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