It's a Matter of Style - The Chicago Manual of Style

The use of small caps for some types of abbreviations may be typographically desirable, but it may no longer be the prescribed style for publications that adhere to the 15th edition of The Chicago Manual of Style due out in August. Instead of AM and PM (in small caps), p.m. and a.m. -- with lowercase and periods -- are the preferred way to indicate time of day. Other abbreviations traditionally set in small caps would now be full caps.

The Chicago Manual Style, which began in 1891 as a style sheet for the University of Chicago Press, is a popular reference guide for writers, editors, proofreaders, copywriters, designers, and, yes, desktop publishers.

Other major changes found in the newest edition of The Chicago Manual of Style, include:

• Set commas, semicolons, periods, and colons in the font of the surrounding text.
• Use 2-em dash rather than 3-em dash to represent a complete missing word or name in running text.
• Abbreviations set in full caps or small caps can omit the periods except U.S. keeps the periods.
• No longer required to use sans serif type for letters representing shapes (e.g., an L-shaped room)
• Three methods of using ellipsis points.

The 15th edition has updates on preparing mathematical copy and editing diagrams, expanded coverage of electronic publications, and a new chapter on American English grammar and usage. See the Manual's Web site at: http://www.chicagomanualofstyle.org/ .