When Barack Obama takes the oath of office today, it will mark the 56th time that a US President has been sworn in for a four-year term since 1789, when George Washington first took the same oath.
During that period, eight Presidents died in office and one resigned; each time, the vice president took the same oath, and completed the four-year term.
Many inaugural events have been added during the past 220 years, but the steps that the President-elect follows to take the constitutionally mandated oath of office, the central event of the inauguration, are essentially unchanged.
The oath will be administered at the US Capitol, in a ceremony on the west front of the building, overlooking the National Mall, as it has been since 1801 when Thomas Jefferson was sworn in. The Supreme Court chief justice first administered the oath in 1797 to John Adams.
The swearing-in ceremony - televised since 1949 - will be attended by Mr Obama's family, past and future Cabinet members, members of the Senate and House of Representatives, Supreme Court justices and many invited guests.
Joe Biden will be sworn in as vice president prior to Mr Obama taking the oath of office, with the oath administered by Supreme Court Justice John Paul Stevens. The vice presidential oath has been part of inauguration ceremonies only since 1937, President Franklin Roosevelt's second inauguration. Previously, the vice president was sworn in at a Senate ceremony because the Constitution designates the vice president as president of the Senate.
Today, Mr Obama will stand before a judge - in this case the Supreme Court Chief Justice John Roberts - and swear the 35-word oath prescribed in Article II, Section 1, of the Constitution:
"I do solemnly swear (or affirm) that I will faithfully execute the office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
When taking the oath, Mr Obama will place his hand on the same Bible President Lincoln used at his first inauguration in 1861. The Bible is currently part of a collection at the Library of Congress. Although a Bible is traditionally used at the swearing in, it is not a constitutional requirement.
The 20th Amendment to the Constitution, adopted in 1933, sets the time and date for the Presidential oath at noon on January 20. In the early days of the nation, when wintertime travel was difficult, inaugurations were held in March.
As in previous inaugural ceremonies, going back to that of George Washington, after taking the oath, Mr Obama will give an inaugural address that outlines the themes for his four years in office.
Other inaugural events
Many elements have been added to the inaugural programme over time. President Obama's inauguration will follow the trend of recent years and feature several days of dinners, balls, receptions and other events focused on the theme of Renewing America's Promise.
This morning, the President and President-elect will meet with outgoing President George W. Bush at the White House and travel to the Capitol together.
Following the noon swearing-in ceremony at the Capitol and the new president's speech, President Bush and first lady Laura Bush will leave Washington as private citizens.
The new President and vice President will make their way from the Capitol back to the White House, where they will view a traditional parade that begins at the Capitol and follows a 2.7-kilometre route up Pennsylvania Avenue and past the White House.
In the evening there will be many formal inaugural balls. Typically, the new first lady's gown for this evening is big news in the fashion world; the dress eventually will make its way into the Smithsonian Institution's collection of inaugural gowns.
Did You Know?
• George Washington gave the shortest inaugural address in history (135 words).
• Chief Justice John Marshall presided over nine inaugurations, from John Adams (1797) to Andrew Jackson (1833).
• In 1865, Abraham Lincoln was the first President to include African Americans in his inaugural parade.
• In 1917, Woodrow Wilson was the first President to include women in his inaugural parade.
• Inauguration Day was changed to January 20, from March 4, in 1933 by the passage of the Twentieth Amendment to the US Constitution. In 1937, Franklin D. Roosevelt was the first president to be inaugurated in January.
• Harry Truman's 1949 inauguration was the first to be televised.
• Ronald Reagan's first inauguration (1981) was the warmest in history and his second was the coldest.
• Robert Frost was the first poet to participate in an inauguration (1961, John F. Kennedy).
• Bill Clinton's 1997 inauguration was the first inauguration to be broadcast live over the internet.
• When January 20 is on a Sunday, the President-elect usually takes the oath of office privately and then repeats the ceremony in public on Monday.