Why Does Am and Pm Stand for?

am - stands for the Latin ante meridiem, translating to "before midday", before the sun has crossed the meridian line. pm - stands for post meridiem or "after midday", after the sun has crossed the meridian line.

Who invented AM and PM?

Anyway, as near as I can tell, the 12-hour clock goes way back to ancient Mesopotamia and Egypt. The Egyptians used a 12-hour sundial to tell time during the daytime and a 12-hour water clock at night. The Romans also used a 12-hour clock.

Is 12 am or pm first?

The first 12-hour period is designated as am. It runs from midnight to noon. The second period, marked pm, covers the 12 hours from noon to midnight.