The US presidential election isn't over yet: Congress must still count the electoral college's votes.

Only then will Barack Obama be officially declared the winner -- a quaint formality, perhaps, but also constitutionally required.

A relic of when Americans traveled on horseback and interstate communication was less than instantaneous, electoral law provides for long delays between the popular vote and the announcement of a result by a joint session of Congress.

That session is scheduled for Friday, at 1:00 pm local time (1800 GMT).

In reality, the outcome was known weeks ago, on election night on November 6.

But the US president is not directly elected by the popular vote.

Instead, Americans in each of the 50 states, as well as the capital, cast their ballots to elect 538 electors: 332 representing Obama and 206 for Romney.

The total figure of electors, and how many are assigned from each state, depends on the population.

On December 17 -- the first Monday after the second Wednesday in December -- these delegates cast their votes in each US state.

Only around half the states require the electors to vote for the candidate they represent, but it is extremely rare for an elector to change candidates or abstain.

These are the ballots that Congress was to count Friday.

And, in the hypothetical situation where no candidate obtained a majority, the House of Representatives would be responsible for choosing a winner, with one vote per state.

Meanwhile, plans were well underway for Barack Obama's second inauguration: a private swearing in on January 20, followed by an immense public ceremony the following day.