Associated Press

For a few hours Thursday, Dallas is the center of American presidential power. As dignitaries gather for the opening of the George W. Bush Library at Southern Methodist University, there will be five current or former inhabitants of the Oval Office: Bush, Barack Obama, Bill Clinton, George H.W. Bush, and Jimmy Carter, all five of the living commanders in chief.

Here also are First Ladies Obama, Laura Bush, Clinton, Barbara Bush, and Rosalynn Carter:

Associated Press

Getting definitive data is tough to come by, but this may tie for the largest such gathering ever. At Richard Nixon's funeral in 1994, Presidents Clinton, George H.W. Bush, Reagan, Carter, and Ford all attended.

Reuters

Both Bushes, Carter, Clinton, and Ford also attended Ronald Reagan's 2004 funeral. Before Reagan's death, there were six current and former presidents, but I haven't found any photographic evidence that they ever met. As Reagan struggled with Alzheimer's disease, he seldom appeared in public from around 2000.

Reuters

Presidents Nixon, Ford, Carter, Reagan, and Bush joined together at the opening of the Reagan Presidential Library in 1991:

The National Security Archive also turned up this great image of President Kennedy, then-Vice President Johnson, Eisenhower, and Truman at late House Speaker Sam Rayburn's 1961 funeral:

National Security Archive

The group convening in Dallas today has met before, including in this January 2009 shot in the Oval Office. But this was in the waning days of the Bush Administration, and Obama had not yet been sworn in.

Reuters

There were five former presidents at the time of Lincoln's inauguration (Buchanan, Pierce, Fillmore, Tyler, and Van Buren), but I find no evidence that they all attended his inauguration, perhaps unsurprising given the division of the country at the time; Tyler would go on to serve briefly in the Confederate legislature (please let me know if I'm wrong!). It also seems plausible that Presidents Washington, Adams, Jefferson, Madison, and Monroe -- perhaps with Adams's son John Quincy -- were together, but never when all were current or former presidents.