Do you capitalize a.m. and p.m.? In formal writing, it is best to lowercase both a.m. and p.m. and retain the periods (as opposed to writing them AM / PM or am/pm); however, should you choose to use a different format, the most important rule is to be consistent throughout your piece. Originally there were two cycles: one cycle which could be tracked by the position of the Sun (day), followed by one cycle which could be tracked by the Moon and stars (night). This eventually evolved into the two 12-hour periods which are used today, one called a.m. starting at midnight and another called p.m. starting at noon. [1] Allied Mastercomputer (also called Adaptive Manipulator or Aggressive Menace ), or AM, the antagonist in the short story and video game I Have No Mouth, and I Must Scream by American writer Harlan Ellison AM means before noon. This is the 12-hour period from midnight to noon. PM means after noon. It covers the 12 hours from noon to midnight. Using numbers from 1 to 12, followed by am or pm, the 12-hour clock system identifies all 24 hours of the day. The abbreviation am stands for ante meridiem, “before midday” in Latin, while pm means post meridiem, or “after midday.” The concept of a 12-hour clock has been around since ancient times. Online shopping for from a great selection at All Departments Store. Explore top New York news and sports with amNewYork. Get the latest city insights and headlines. Your premier NYC news source! AM stands for ante meridiem, which translates literally to before midday. PM stands for post meridiem, which means after midday. These historical terms reflect how humans have used the sun’s position to track the passage of time for centuries. As we said above, AM stands for “ante meridiem” or before noon in Latin, so it is used to describe the first portion of the day – the first 12 hours. On the other hand, PM stands for “post meridiem” or after noon in Latin and it’s used to describe the second portion of the day – the final 12 hours. What Do A.M. and P.M. Stand For? The abbreviation a.m. is short for the Latin ante meridiem, meaning before noon, referring to the chunk of time from midnight until noon. The abbreviation p.m. is short for the Latin phrase post meridiem, which means after noon. Srinivasan.Clicks/Shutterstock.