What Do A.M. and P.M. Stand For?

One of the inconvenient things about breaking your day into two 12-hour chunks numbered 1-12, which turn over at midday and midnight, is that it becomes necessary to tell somebody which chunk you’re talking about — are we meeting at eight in the morning or eight at night? For this reason the two periods are called a.m., short for the Latin term ante meridiem, or “before midday,” and p.m., which is short for post meridiem, or “after midday.”

But why should we have to complicate things with a 12-hour clock when the 24-hour clock seems so much more straightforward and convenient? The answer to this has a lot to do with early clocks.

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