Since the first "red phone" in 1963 between the US and the Soviet Union, many countries including the two Koreas have set up communication hotlines between leaders to ease dialogue in times of crisis.
In October 1962, the US detected Soviet missiles in Cuba as tensions mounted over nuclear conflict.
In the confrontation between John F. Kennedy and Nikita Khrushchev, the absence of direct communication enabled rumours on the intentions of both, while messages between Moscow and Washington took hours to be delivered.
In this handout image, a South Korean government official checks the direct communications hotline to talk with North Korea on 3 January, 2018. Source: Getty Images
Nuclear tensions
A letter from 26 October, 1962, in which the Soviets laid out a possible way out of the face-off, was received by the US ambassador to Moscow at 9:42am Washington time, but after being translated, transmitted and encrypted, it was after 9 pm by the time the note was in the hands of the State Department.
To speed up communication and reduce the risk of nuclear war, the two governments put in place a "red phone" on 30 August, 1963. In reality, it was neither red nor a telephone but a cable line that could transmit written messages.
In the 1970s, a satellite phone line was added, which also allowed for documents, maps or photos to be shared between the two governments.
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In 1994, a new "red telephone" allowed the defence secretaries of the two countries to be contactable nearly all the time.
The White House and the Pentagon have always kept secret exactly how many times it has been used.
Leaders employed the hotline during the Arab-Israeli wars of 1967 and 1973, and again with the Soviet invasion of Afghanistan in 1979.
'Red phones' worldwide
Other lines of direct communication were subsequently installed between Moscow and European capitals.
In 1996, China established a "red phone" with Russia, and then with the United States two years later. In 2005, India and Pakistan did the same.
A hotline between South Korea and North Korea was agreed to as part of a 1972 joint communique between the two nations and went into operation in August that year.
The phone and fax lines have been disconnected and reconnected several times since then, depending on the state of relations. The hotline was reopened on Wednesday after a shutdown of nearly two years.
In September 2011, the United States proposed opening a direct line with Iran to avoid a possible conflict erupting over the country's disputed nuclear program. Tehran declined the offer.
In June 2013, China and Vietnam opened a direct communication channel to manage their territorial conflict in the South China Sea.
The term "hotline" is today used to characterise all forms of direct communication between countries or sensitive authorities, over issues such as security cooperation.
Japan decided in 2013 to install a hotline between its security council and Washington and London.
