They may be the most familiar three-notes to generations of Americans—the NBC chimes hit the notes G-E-C.

But there is a long-forgotten part of the history of NBC—the fourth chime.

The background goes back to the roots of NBC Radio. By all accounts, the first use of the three-note chime was on November 29, 1929, on the Red and Blue Radio Networks. (Long before cable television, NBC ran the two radio networks—although the Blue network was eventually spun off and became ABC).

There is a dispute as to who actually came up with the idea for the chimes, but they solved a very practical problem: how to cue local affiliates for a station break. So the studio announcer hit the chimes every half-hour, and listeners would hear station identifications from around the nation, such as "This is WEAF, New York" or "This is WJZ Baltimore."

The chimes became an integral part of the story of NBC News, long before satellites, smart phones and the Internet.

Several years after the three-signal chime was first used, a fourth was added, but was only deployed in extraordinary circumstances.

Here's how it was described in a book called "The Fourth Chime"—a 174-page history of NBC News from the Japanese invasion of Manchuria in 1931 through D-Day, published by the National Broadcasting Company in 1944 and long out-of-print:

"The Fourth Chime, a note added to the familiar NBC three-chime signal, is the exclusive property of the Newsroom of the National Broadcasting Company; rings out from the NBC Newsroom only when events of major historical importance occur.”

The book continues: “Contrived originally as a confidential ‘alert’ to effect the immediate gathering of members of the NBC news staffs, engineers and other operating personnel responsible for broadcasting the news to the people, NBC's Fourth Chime has come to be significantly identified with every major news break of the past seven years."

So for staffers, who probably had to leave their radios on all night, it was an "all-call" to get to work.

The fourth chime was first used when the Hindenburg exploded over Lakehurst, New Jersey, in 1937. It also rang out for the Munich crisis of 1938, the morning of the Japanese attack on Pearl Harbor, then again early in the morning of June 6th, 1944, after Allied forces landed on the northern coast of France.

Minutes after the four chimes were sounded, NBC newsman Robert St. John made the announcement from the New York newsroom: "Men and women of the United States, this is a momentous hour in world history. This is the invasion of Hitler's Europe—the zero hour."

As the "Fourth Chime" puts it, St. John's announcement "began the most exciting twenty-four hours of news coverage in the history of broadcasting."

From the first word of the invasion, NBC launched into non-stop coverage, including what appeared to have been the first eyewitness account from reporter Wright Bryan, who was embedded as a pool reporter with U.S. paratroopers.

Decades later, the days of using the fourth chime to bring NBC News employees to work during major news events are long gone. But one thing hasn't changed: We are still producing the news from 30 Rockefeller Plaza.