The United State constitution offers the vice president a limited role.
Washington, United States:
There is an old joke that goes something like this: A woman had two sons. One went off to sea, the other became vice president of the United States. Neither was ever heard from again.
That more or less summed up the way many Americans long felt about the nation's number two job -- not that relevant.
But in recent decades, the narrative has shifted. Vice presidents have become more visible, and have much better access to the Oval Office.
As Donald Trump and Hillary Clinton battle for the White House in November, and choose running mates -- Trump has selected Indiana Governor Mike Pence -- the debate on the role, and usefulness, of an American vice president is raging again.
The US constitution offers the vice president a limited role: it says he should preside over the Senate, but only vote in the event of a tie.
Of course, the other major role is to take over as president, should the sitting president be unable to serve or resign. Examples are Lyndon B. Johnson's accession to the presidency after the assassination of John F. Kennedy, and Gerald Ford's after Richard Nixon resigned over the Watergate scandal.
Over the years, people have described -- sometimes with humor -- the frustrations generated by this singular position.
"It's like being naked in the middle of a blizzard with no one to even offer you a match to keep you warm -- that's the vice presidency," Hubert Humphrey said in 1969, just a few months after leaving the job.
"You are trapped, vulnerable and alone, and it does not matter who happens to be president."
For a long time, "the office was sort of a political dead end," says Joel Goldstein of Saint Louis University School of Law.
Before George H.W. Bush won election to succeed Ronald Reagan in 1989, the last vice president to be elected to the highest office in the land was... Martin Van Buren, in 1836.
But the job has changed dramatically.
"After World War II and with the Cold War and the nuclear age, presidents saw that there were a lot of demands on them and that people wanted the VP to be in the loop in case the president died," Goldstein explains.
Harry Truman, who assumed the presidency in 1945 after the death of Franklin D. Roosevelt, learned only after taking office of the existence of the Manhattan Project -- the code name for America's nuclear weapons research program.
For a long time, the vice president was even physically distant from the seat of executive power -- his office was in the Senate.
'The most insignificant office'The turning point for the vice presidency came under Jimmy Carter (1977-81), who made space for his number two, Walter Mondale, in the West Wing of the White House.
Since that time, no one has changed the layout: the vice president has an office between the chief of staff and the national security adviser.
"That proximity to power has had practical but also symbolic importance," Goldstein notes.
In practical terms, the modern American vice president is a "super adviser" to the president. He is part of the inner circle.
For President Barack Obama, Joe Biden is that guy. In the now-classic photo of the White House Situation Room on the night in 2011 when Osama bin Laden was killed in a US raid, Biden was right next to Obama.
Reagan largely relied on the foreign policy knowledge of George H.W. Bush, a former CIA director and US envoy to the United Nations.
Bill Clinton counted on Al Gore to help him in many political battles.
During George W. Bush's presidency, Dick Cheney's stint as the number two blazed a new path -- a troubling path, for many observers.
Cheney's influence, particularly after the September 11 attacks, raised questions about how much he was deciding, and how much Bush was deciding.
"Dick Cheney said I'm the worst president of his lifetime, which is interesting, because I think he's the worst president of my lifetime," Obama joked at the 2015 White House Correspondents' Dinner.
Biden, who has a genuinely close relationship with Obama, has gone to war for his boss on several fronts for the past eight years, including economic recovery plans, withdrawal of US forces from Iraq and the national debate on gun control.
"I literally get to be the last guy in the room with the president. That's our arrangement," Biden said in 2012 with his trademark grin at a campaign rally.
The contrast between the days of America's Founding Father and today is profound: John Adams, the country's first vice president, bitterly complained about his status in a letter to his wife Abigail.
"My country has in its wisdom contrived for me the most insignificant office that ever the invention of man contrived or his imagination conceived," he wrote.