AP style update
The stylebook committee has been busy lately.
This update is to the "caps, periods" section of "abbreviations and acronyms." It now says:
CAPS, PERIODS: Use capital letters and periods according to the listings in this book. For words not in this book, use the first-listed abbreviation in Webster's New World College Dictionary. Generally, omit periods in acronyms unless the result would spell an unrelated word. But use periods in two-letter abbreviations: U.S., U.N., U.K., B.A., B.C. (AP, a trademark, is an exception. Also, no periods in GI and EU.)The old entry (from my 2003 edition) said to use Webster's New World Dictionary and then said, "If an abbreviation not listed in this book or in the dictionary achieves widespread aceptance, use capital letters. Omit periods unless the result would spell an unrelated word." Period.
Use all caps, but no periods, in longer abbreviations and acronyms when the individual letters are pronounced: ABC, CIA, FBI.
Use only an initial cap and then lowercase for acronyms of more than six letters, unless listed otherwise in this Stylebook or Webster's New World College Dictionary.
To lowercase acronyms of more than six letters is good, but I wonder why six. At the very least, I'd go with six or more. And I would prefer five or more, which I believe is the rule at the New York Times and the Wall Street Journal.
Want more? There's a related thread at Testy Copy Editors.