Seven states are expected to play a key role in electing the next president of the United States: Arizona, Wisconsin, North Carolina, Georgia, Michigan, Nevada and Pennsylvania.

So what is a battleground state and why does its vote matter?

There are 538 Electoral College votes divided among the 50 states and the District of Columbia. Each state receives at least three votes depending on the size of its congressional delegation.

Congressional seats are allocated to each state based on its population. So the smallest states get three Electoral College votes (they have two senators and one member of Congress). Washington, DC also gets three. The most populous states get many more: California gets 54 electoral votes (it has two senators and 52 congressional districts), Texas has 40, Florida has 30, New York has 28, and so on.

In almost all states, the winning candidate receives all the votes. The candidate who receives 270 or more electoral votes becomes president.

Some states are historically blue (where Democrats dominate) or red (where Republicans dominate). In battleground states, voters do not always vote for the same party, but battleground states can change from election to election.

The larger battleground states like Pennsylvania have more seats in Congress and are therefore particularly important to a candidate's path to 270. And if a candidate wins three or more battleground states, he or she is more likely to win the election.

Therefore, the seven contested states could decide who becomes president.

Read more about how the US election works.