Yesterday we had a chance to see five living presidents on stage at the dedication of the George W. Bush Presidential Center: Jimmy Carter, George H.W. Bush, Bill Clinton, George W. Bush and Barack Obama. Looking back in history, there are relatively few times when we have had five living presidents and only a few years when we have had six living presidents. But such occasions have been much more frequent in the last quarter century than ever before.

When has this happened before?