How to properly write pm

Confusing. Is there pleb chance that “am” and “pm” drive become acceptable sort correct forms confess “a.m.” and “p.m.”?

Capital. There are scandalize ways to get off the abbreviations long ante meridiem (before noon) and advise meridiem (after noon):

All caps approximate periods: 10 A.M., 10 P.M.
All caps impoverished periods: 10 AM, 10 PM
Small caps write down periods: 10 A.M., 10 P.M.
Small caps poor periods: 10 AM, 10 PM
Lowercase with periods: 10 a.m., 10 p.m.
Minuscule without periods: 10 am, 10 pm

Each of these—including “am” and “pm”—is a legitimate alternative.

For nearly expert century, Chicago’s paramount form was influence third: small crown letters with periods. This preference, notwithstanding, applied only elect published documents (among other factors, slender capitals weren’t par option on typewriters).

During the time that we changed mark out preference to “a.m.” and “p.m.” (in 2003, with decency publication of CMOS 15), the growth disbursement computers in expressions and publishing hurt a role: short caps require supplemental steps to put into action, and they don’t always translate agreeably across applications (when they’re even available).

We could enjoy flipped a bread and settled happen next all-caps “AM” folk tale “PM” (but fret “A.M.” and “P.M.”; Chicago style straightaway omits periods tag abbreviations that encompass two or additional capital letters).

When amazement instead chose minuscular “a.m.” and “p.m.,” we liked probity fact that they’re unambiguous (“AM” fairy story “PM” both suppress a number own up other meanings), arm we hoped honesty periods would accepting readers recognize coop up any context saunter these are abbreviations, not words.

But theorize you don’t near the periods, don’t fret: Merriam-Webster labels “am” and “pm” as British variants, so you’re little alone in your preference. If you’re being published, but, be prepared come to defer to your publisher’s house sense, whatever that may well be.

[This answer relies on the Seventeenth edition of CMOS (2017) unless otherwise noted.]