One of the oddest aspects of Election Day in the United States is that there's no standard way for Americans to vote. Different states employ different rules on who can vote and when. What's more, voting technology can vary wildly.

For instance, this map from Mike Orcutt and looks at the different methods that voters in different states use to cast ballots. Some states, particularly in the Northeast, stick with paper ballots and punch cards. Other states have electronic machines with a paper trail that voters can review for themselves. Still others states, like Maryland and Virginia, use electronic machines with no paper trail:

Electronic voting machines became popular in 2002, after the Help America Vote Act required every polling place to have a handicapped accessible voting system. Many states decided to use their funds to switch to touchscreen systems. As a result, more than 25 percent of Americans are expected to cast their ballot by electronic machine in this election.

But as Pamela Smith of Verified Voting explained in an interview, not all of these electronic machines leave paper trails that can ensure the integrity of the system. That's troubling since if there's a discrepancy in the numbers, these machines are more difficult to audit. "At the end of the day, you need to be able to reconcile the number of people who came into a polling place with the number of votes cast," Smith says. "If it's off by a lot, there's a real problem there!"

Problems aren't unheard of. In 2004, Carteret County in North Carolina simply lost 4,500 votes because of a memory problem in the electronic machines. Similarly, a large report (pdf) from Verified Voting, Rutgers Law School, and Common Cause documented more than 1,800 complaints about electronic voting machines in the 2008 election and more than 300 about the 2010 election.

The report recommended that more states with electronic voting machines keep a paper trail -- a practice that some swing states like Ohio and North Carolina have adopted. Others, like Florida, are pushing for that system.

Electronic machines can also be more likely to cause delays and longer lines at a polling place. If a machine's touchscreen is badly calibrated — so that it highlights "Mitt Romney" when you try to vote for "Barack Obama" — then a technician will need to come in and fix it. That slows things down for everyone. By contrast, if an optical scanner for paper punch cards is busted, that's not a big deal — people can still vote, it will just get counted later.

Some of these problems do come down to money, Smith says. Many states are using technology that's more than a decade old, and funds are scarce to upgrade to systems that leave paper trails or develop contingency plans for broken-down machines. Most of the money from the Help American Vote Act has largely been spent. "A lot more jurisdictions would like to move to an evidence-based system, but there aren't always the resources to make that switch," Smith says.