Capitalizing AM and PM is a question that the Editor's Manual asks.

The abbreviations may be capitalized in headings, signs, and notices.If capitalized, periods are omitted, but the letters of the abbreviations are followed by periods.If you use these abbreviations in a document, consider using small caps to make it easier to read.

The two 12-hour periods of the day are referred to by the abbreviations for time.

The exact time can be emphasized with numerals.In official and academic documents, these two abbreviations are used.In informal communication, we use words.

There are periods after each of the letters in running text.Is it okay to start a sentence with but?In headings, signs, and notices, the abbreviations may be capitalized.

The Chicago Manual of Style and the AP Stylebook suggest using lowercase instead of capital letters.Chicago suggests using small capital letters in running text to make it more readable.

The periods when you use the abbreviations are omitted.

Periods are not always used in British abbreviations.We serve thinly sliced cucumber sandwiches at 4 pm.

Convention is more important than a fixed set of rules in determining whether an abbreviation is capitalized.Many abbreviations in the world of information technology are capitalized, while the sciences use a mix of capital and lowercase letters.Stay consistent in usage throughout the document by following the convention of your field.

There is a fair use policy for the contents of this page.You have to link back to this page if you quote this article.The section heading has a link icon beside it.Our Terms of Use can be found here.