The annual State of the Union address is the keynote speech by the president to Congress in which he sets out his agenda for the next year and highlights his accomplishments to the American people.
It is a requirement of the US constitution that the president "shall from time to time give to the Congress information of the State of the Union, and recommend to their consideration such measures as he shall judge necessary and expedient".
But the requirement has been interpreted differently over the past 200 years, with the custom of sending only a written statement prevailing for much of the 19th Century.
The first State of the Union speech was delivered by President George Washington in 1790 in New York, the first US capital.
But the third President, Thomas Jefferson, objected to appearing in person - saying it was too much like an imperial or king's speech, and for the next 100 years presidents sent a written message to Congress that was then read out for them.
ABRAHAM LINCOLN
President Woodrow Wilson, who led the United States into the First World War, was the first president since John Adams to address Congress directly, in 1913, urging reform of high US tariff barriers.
But as the power of the presidency has increased in the last century, the importance of the State of the Union speech has grown.
And that power has been reinforced by the role of television and radio, which has allowed the president to reach out to the American people.
Impact of the media
Calvin Coolidge, in 1923, was the first president to broadcast his speech on the radio, but it was Franklin Delano Roosevelt, who took office in 1932, who was the master of the radio address.
FRANKLIN ROOSEVELT
His "Four Freedoms" State of the Union speech in 1941 outlined the rights that Americans would soon be fighting for as the United States edged towards joining World War II.
By 1947, under President Truman, the State of the Union speech was televised - and by the 1960s it was moved to a 2100 evening time slot to gain a greater television audience.
In later years President Lyndon Johnson used the State of the Union to launch his Great Society programme to end poverty in America, while Bill Clinton promoted his stalled plan for healthcare reform.
But two of the most memorable State of the Union speeches occurred in the 19th Century.
Making history
In 1823, President James Monroe outlined the "Monroe Doctrine", which called for the European powers to abstain from interference in the foreign affairs of the Americas, and endorsed the movement for independence by South American nations from Spain.
GEORGE W BUSH
And in 1862, President Abraham Lincoln expressed his desire for the Civil War to lead to the emancipation of the slaves, which was later realised in his Emancipation Proclamation, which freed the slaves in the Southern states in rebellion from the Union.
The State of the Union speech has always been flexible - addressing both domestic and foreign policy needs.
In 2002 and 2003 President George W Bush used his State of the Union speech to argue the moral case for war against Iraq, labelling the regime part of the "axis of evil" that threatened world peace.
He has also used the speech to launch surprise new initatives, such as the $15bn Emergency Plan for Aids Relief, and to reinforce domestic priorities such as tax cuts and healthcare reform.
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