During the height of the Cold War, the United States and the Soviet Union established a direct communications link to allow their leaders to contact one another in the event of a nuclear crisis or other emergency. This Washington-Moscow hotline has since featured in countless novels and films such as 1964’s “Dr. Strangelove,” but contrary to its depictions in pop culture, it never took the form of a red telephone. In fact, it never involved phone calls at all.

The Washington-Moscow hotline was first proposed in the 1950s, but the idea didn’t gain traction until 1962’s Cuban Missile Crisis, when the Americans and Soviets found that their diplomatic messages often took several hours to reach one another. Fearing that any further mishaps might trigger an accidental nuclear war, the two superpowers met in Geneva the following year and signed a “Memorandum of Understanding Regarding the Establishment of a Direct Communications Line.” On August 30, 1963, the new system went live. Rather than a telephone link, which presented the possibility of miscommunications, the hotline consisted of teletype machines that allowed the two countries to send written messages to one another via transatlantic cable. The Soviet system was located in the Kremlin, but the American version was always housed in the Pentagon, not the White House. Satellite links were later added to the hotline during the Nixon administration, and in 1986, it was upgraded to include high-speed fax capability. The most recent overhaul came in 2008, when the system switched to email.

While there is no evidence that the hotline was ever used to avert a nuclear disaster, it often played a key role in U.S.-Soviet relations. In 1967, Lyndon B. Johnson became the first president to use the system when he negotiated with Soviet leader Alexei Kosygin during the Six Day War, a brief conflict between Israel and several Arab states. Richard Nixon later used it for similar purposes during 1971’s Indo-Pakistani War and 1973’s Yom Kippur War, and Jimmy Carter famously hopped on the hotline to object to the Soviet invasion of Afghanistan in 1979. The hotline’s last crisis uses came during the Reagan administration and the dying days of the Cold War, but it still exists to this day. To ensure that the system will function in the event of an emergency, Russian and American technicians continue to send test messages to one another once every hour.