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.
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.
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.
Presidents Nixon, Ford, Carter, Reagan, and Bush joined together at the opening of the Reagan Presidential Library:
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:
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.
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.
Groups of four presidents, however, are more common. Here are Presidents Kennedy, Truman, Johnson (in back), and Eisenhower at Eleanor Roosevelt's 1962 funeral.
Herbert Hoover, whose record for longest ex-presidency was only broken by Jimmy Carter in September of 2012, remained involved in national politics after losing to Franklin Roosevelt in 1928, and in fact became close friends with Harry Truman. Here are Dwight Eisenhower, then still in the military, Truman, and Hoover in 1947.
But the crown for most gathered actually belongs to First Ladies. Here's a 1994 photograph of First Lady Hillary Clinton (far right) along with (from left) Lady Bird Johnson, Betty Ford, Rosalynn Carter, Nancy Reagan, and Barbara Bush.
At the Washington Post, David Beard has a nice round-up of photos unearthed by historian Michael Beschloss.