“Three Famous Notes of Broadcasting History” by Bill Harris:

Almost anyone who ever listened to radio has at some time or other, heard the famous three note chime that has been the long time trademark of NBC. These chimes were used on the hour and half-hour to announce station breaks on the network.

I became interested in the history of the chimes after discovering a book at the library titled The Fourth Chime by NBC, printed in 1944. I had never heard of a “fourth” chime and my curiosity was aroused. I checked out the book to find out more about this extra chime. However the book told very little about the fourth chime, but dealt mainly with the role NBC played in the reporting of special world-wide news events, primarily during World War II.

A quick and fascinating read about a portion of the broadcasting history of the United States. Like the author, I think the portion on the “fourth chime” is the most fascinating part.

There’s a great Wikipedia article on the same, though it appears to be largely sourced from the article I linked above.