The Complete Guide to A. M. and P. M. : Definitions, Origins, and Everyday Usage
By Sarah Jenkins
Different publications apply strict, contrasting rules to how these Latin abbreviations should appear on the printed page:
- Associated Press (AP) Style: Prefers lowercase letters with periods and a preceding space: 9 a.m. and 7:30 p.m.
- The Chicago Manual of Style (CMOS): Recommends either lowercase with periods (a.m. / p.m.) or capitalized small caps without periods (A.M. / P.M.).
- British Style (BBC, Oxford): Frequently drops the periods entirely, opting for lower-case compact notation (9am, 7.30pm).
Modern digital clock formats on smart home devices, smartphones, and car dashboards often omit periods altogether due to display constraints. They display bold capital abbreviations ("AM" / "PM") or simple two-character indicators to conserve screen real estate. When writing professionally, picking one consistent standard, and never combining redundant phrases like "8:00 a.m. in the morning", remains the editorial baseline.
Sarah Jenkins is a veteran tech journalist with over 12 years of experience covering artificial intelligence, mobile innovations, and digital ethics. Her insights have appeared in leading technology publications worldwide.
