a.m., p.m.

Chicago Style recognizes two different ways of writing the “a.m.” and “p.m.” abbreviations. This first way is lowercase “a.m.” and “p.m.” The way requires periods and is preferred in Chicago Style. The second way is with small capitals “am” and “pm” This second way can appear with or without periods, making “am” and “a.m.” both acceptable. All of the following examples are acceptable in Chicago Style:

  • Everyday, I eat breakfast at 10:33 a.m. and dinner at 5:34 p.m.
  • I nearly missed my 7:00 am bus.
  • Your test will begin at 10:10 a.m. and conclude at 12:50 p.m.

While it is acceptable to write “a.m.” and “p.m.” a few different ways, you should always be consistent. Pick one way and stick with it. For example, you should never punctuate a sentence as follows,

  • I start work at 8:00 am and leave at 5:00 p.m.

Instead it should be writen one of the following ways.

  • I start work at 8:00 am and leave at 5:00 pm
  • I start work at 8:00 a.m. and leave at 5:00 p.m.
  • I start work at 8:00 a.m. and leave at 5:00 p.m.

Also, in Chicago Style, even hours (10:00 a.m.) require two zeros.

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