Jimmy Carter, the 39th president of the United States, has lived long enough to overlap with 17 other commanders-in-chief.

Longtime friends, family and fans of Jimmy Carter milled around his hometown of Plains to celebrate his 100th birthday on Tuesday, the first time an American president has lived a full century and the latest milestone in a life that took the Depression-era farmer's son to the White House and across the world as a Nobel Peace Prize-winning humanitarian and advocate for democracy.

Back in 1976, Carter changed the way presidential candidates are nominated and elected. He went on to provide models of what to do – and what not to do.