The United States is holding the quadrennial presidential inauguration ceremony on Tuesday, as Barack Obama will be sworn in as the country's first black president.

The ceremony has been through dramatic changes in form and process since it was started from George Washington's presidency, but remained one of the most significant political events in the country, where all presidents-elect were required to make an oath following the Constitution with the Chief Justice and the whole nation as witnesses.

When Washington was sworn in as the first U.S. president in 1789, the ceremony took place on April 30 in New York, which used to be the country's capital. As it took as long as four months to count the presidential votes at that time, the inauguration date had been set on March 4 for most of the country's history. It was not until 1937 that the inauguration day was finally moved ahead to Jan. 20.

In 1801, the U.S. federal capital was relocated at Washington, D.C., so did the inauguration ceremony. In 1981, Ronald Reagan had his inauguration on the West Front of the U.S. Capitol, which has since replaced East Portico as the stage for the ceremony. All the presidential swearing-ins were held outdoors except twice that happened to William Howard Taft in 1909 and Reagan in 1985, when the weather was so cold that public health and security were at risk.

According to the fixed process, the vice president-elect takes the oath of office first on the Capitol's West Front, and the military band plays four ruffles and flourishes and "Hail, Columbia" following the oath.

At noon, the president-elect is sworn in by the Chief Justice, using the form mandated in the Constitution that goes like "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."

Immediately after the oath, the military band plays four ruffles and flourishes and "Hail to the Chief," followed by a 21- gun salute. Then the president delivers an inaugural address including his ideas on the new administration.

In the meantime, outgoing President and the First Lady depart from the Capitol to begin their post-White House life.

As another grand activity in the inauguration day, the presidential parade provides an opportunity for more people to witness the president and vice president as well as their family members walking to the White House along the historical route of the Pennsylvania Avenue to begin their leadership.

The parade, which is going to be the 56th one this year, started in 1805 when Thomas Jefferson began his second term. In the U.S. history, there was only one time, when Ronald Reagan reassumed his office in the White House in 1985, that the swearing- in was not followed by the parade due to severe weather.

Other inauguration-related events usually last for several days before and after the inauguration day, including official balls, parties, concerts, and religious services, which involve millions of people.

However, the inauguration activities also bring huge challenges to transportation and security in Washington, D.C. and its suburb areas. The Armed Forces, Secret Services, Capitol police, D.C. police and other federal law enforcement agencies are required to cooperate in security protection and road closure, among others.

(Xinhua News Agency January 20, 2009)