AM runs from midnight to noon and PM runs from noon to midnight. Learn what the letters stand for and why 12 AM and 12 PM are easy to mix up. 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. 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. 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 Learn what is am and pm in the 12-hour clock system. AM means before midday and PM means after. Master time telling with simple rules and examples. 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. The meaning of AM is present tense first-person singular of be. 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] 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. Online shopping for from a great selection at All Departments Store.