Voters in the United States go to the polls on Tuesday to elect a new president -- chief executive of the US government and commander-in-chief of the US military.
The process of the US presidential elections is divided into four stages -- the prenomination phase, the national conventions, the general election campaign and the Electoral College phase.
In the prenomination phase, usually from February to June in every presidential election year, candidates compete in state primaries and caucuses, the two methods by which the states choose their delegates to the national conventions.
Primaries, held by most US states, are open to all registered voters. As with general elections, voting is done through a secret ballot. Voters may choose from among all registered candidates.
Most states hold presidential preference primaries in which the actual presidential candidates' names appear on the ballot. In other states, only the names of convention delegates appear on the ballot. Delegates may state their support for a candidate or declare themselves to be uncommitted.
In some states, delegates are bound or "pledged" to vote for the primary winner in voting at the national convention. In other states some or all delegates are "unpledged" and remain free to vote for any candidate they wish at the convention.
Caucuses are simply meetings, open to all registered voters of the party, at which delegates to the party's national convention are selected.
As in the primaries, the caucuses can produce both pledged and unpledged convention delegates depending on the party rules of the various states.
The national conventions are held between July and August, when political parties in the United States will choose their presidential candidates. At the conventions, the presidential candidates are selected by groups of delegates from each state.
After the national conventions, the general election campaign will begin and last eight to nine weeks, during which the major party nominees, as well as any minor party or independent contenders, compete for votes from the entire electorate, culminating in the popular vote on election day in November.
The general election campaign is followed by the Electoral College phase, in which the president and vice president are officially elected.
On the first Tuesday following the first Monday in November in every presidential election year, registered voters in the country 's 50 states and the District of Columbia vote for president and vice president.
However, Americans do not vote directly for presidential candidates. Instead, after ballots are tallied in each state, state representatives, called electors, vote based on the state tallies in an Electoral College, a system which has been operating since 1788.
According to the system, a state's number of electoral votes equals the number of senators and House representatives combined from that state. As the number of House representatives is based on the size of population of each state, the number of electors varies from state to state.
In all but two states -- Nebraska and Maine -- winner of the popular vote (the total number of votes cast by people in a given state) takes the state's total allotment of electoral votes.
Electors meet and officially vote for president and vice president on the first Monday following the second Wednesday in December each presidential election year. A majority of votes are needed for a candidate to be elected.
Since there are altogether 538 electors, a minimum of 270 electoral votes is necessary to win the Electoral College.
If no candidate receives a majority, the House of Representatives -- one of two houses of the US Congress -- must determine the winner from the three candidates who received the most votes in the Electoral College.
The president and vice president take their oath and assume office on Jan. 20 following the election. The president is elected for a term of four years and may be re-elected once.