Backgrounder: U.S. presidential debates

WASHINGTON, Sept. 26 (Xinhua) -- U.S. presidential candidates Barack Obama and John McCain will hold their first debate Friday night in Oxford, Miss.

While the first nationally-televised presidential debate was not held until 1960, several other debates are considered predecessors to the presidential debates, including the debates between Abraham Lincoln and Stephen Douglas in middle of 19th century.

The first nationally-televised general election presidential debate was held on Sept. 26, 1960, between John F. Kennedy and Richard Nixon.

But no general election debates at all were held for the elections of 1964, 1968 and 1972.

It wasn't until 1976 that presidential debates became a formality of the presidential election.

Some of the presidential debates can feature the candidates standing behind their podiums, or in conference tables with the moderator on the other side.

Depending on the agreed format, either the moderator or an audience member can be the one to ask a questions.

The nonpartisan League of Women Voters (LWV) organized presidential debates in 1976, 1980 and 1984.

Since 1988, two major political parties assumed control of organizing presidential debates through the Commission on Presidential Debates (CPD).

Some criticized the exclusion of third party and independent candidates of the debates but it is hard for any third party or independent candidate to pass the threshold of 15 percent of approval rating to get a ticket to the debate.

The only exception was in 1992, when a third-party candidate, billionaire Ross Perot joined a debate with George H. W. Bush and Bill Clinton.

This year, McCain and Obama will hold three debates on Sept. 26,Oct. 7 and Oct. 15 while the vice presidential nominees Republican Sarah Palin and Democrat Joe Biden will face-off on Oct. 2.