US election 2016: How does it all work?

In January 2017, the most powerful nation on earth will have a new leader, after a drawn out and expensive campaign - but how does a US presidential election work?
When the US picks its president, it is not only choosing a head of state but a head of government and a commander-in-chief of the largest military on the planet.
It's a big responsibility. So how does the process work?

Who can be president?

Technically, to run for president, you only need to be "a natural born" US citizen, at least 35 years old, and have been a resident for 14 years. Sounds easy, right?
In reality, however, every president since 1933 has been a governor, senator, or five-star military general. And that's before you even consider getting a party nomination and securing national media attention.
In this 2016 election, at one stage there were 10 governors or former governors and 10 who are or were senators, although many have since dropped out.
One person is nominated to represent the Republican and Democratic parties in the presidential election.

How to become the president of the US

A series of elections are held in every state and overseas territory, starting in February, which determine who becomes each party's official presidential candidate.
The winner of each collects a number of "delegates" - party members with the power to vote for that candidate at the party conventions held in July, where candidates are formally confirmed.
The more state contests a candidate wins, the more delegates will be pledged to support them at the convention.
As President Barack Obama cannot run again, both parties are holding competitive primaries this year.
The Republican candidate will need 1,237 delegates to win a majority, while the Democratic contender must secure 2,383.

What are the key dates between now and the election?

The first votes were cast in Iowa on 1 February - it was the first US state to have a contest (although in Iowa's case, it's a caucus, which is a vote of people present rather than through a ballot).
Other early states include New Hampshire on 8 February and South Carolina, which means they have presidential candidates visiting them for months on end.
On 1 March, a dozen states pick their presidential nominees, so it's called Super Tuesday. In 2016, the primaries held on 15 March, including Florida, Ohio and North Carolina, could be significant because so many delegates are up for grabs.
By the end of April, most states have cast their votes and in most election campaigns, it's clear by then who each party has picked as their presidential candidate. But it's not official until the party conventions in July.
If you're still with us, you'll be glad to know the real campaigns haven't even started yet.
That happens after the summer, when the two candidates hold a manic, mammoth journey whizzing across the country to make their case.
There are three televised presidential debates in the last six weeks before - finally - votes are cast on Tuesday, 8 November.

How does the vote in November work?

The candidate with the most votes in each state becomes the candidate which that state supports for president.
It's all down to a system called the electoral college, a group of people who choose the winner - 538 of them, in fact. Just half of them - 270 - are needed to make a president.
But not all states are equal - California, for example, has more than 10 times the population of Connecticut, so they don't get an equal say.
Each state has certain number of these "electors" based on their population in the most recent census (it so happens that it's the same number of districts in a state, plus two senators).
When citizens vote for their preferred candidate, they're actually voting for the electors, some of which are pledged to one candidate, some for another.
But here's where it gets interesting. In almost every state (except Nebraska and Maine), the winner takes all - so the person who wins the most electors in New York, for example, will get all 29 of New York's electoral votes.
In the race to get to the magic number - 270 - it's the swing states that often matter most.

What are swing states?

So, we've got two candidates, both in a race to get to 270 electors by winning whole states at a time.
Both parties think they can bank on certain states, big and small. Republicans will count on Texas, and not waste their money campaigning to a great extent there. Similarly, California is likely to sit in the Democrats' column.
The others are known as "swing states" - where it could go either way. Florida in particular, with its 29 votes, famously decided the 2000 election in favour of George W. Bush, who lost the popular vote nationally but, after a Supreme Court case, won the electoral college.
Other swing states include: Ohio, Virginia, Colorado, North Carolina, Nevada.

When does the new president start work?

In the days and weeks after the election - if the vote is decisive - the victor will assemble a cabinet and begin crafting a more thorough policy agenda.
Meanwhile, the departing "lame duck" president works to shape his legacy and begins packing up his belongings.
Under the US constitution, the president is inaugurated on 20 January of the year following the election.