On Election Day, it may seem like it’s too late to register to vote for the first time, but about half of states allow potential voters to do just that.

About 20 states and two territories offer same-day registration, allowing voters to complete their registration and cast their ballot at the same time. One state, North Dakota, does not require people to register at all, so any eligible voter can show up and vote on Election Day.

The states and territories with same-day registration are Alaska, California, Colorado, Connecticut, Hawaii, Idaho, Illinois, Iowa, Maine, Maryland, Michigan, Minnesota, Montana, Nevada, New Hampshire, New Mexico, Puerto Rico, Rhode Island, Utah, Vermont, Virginia, Washington, Wisconsin, Wyoming and Washington, DC

Voters in the remaining states can only vote on Election Day if they register before their state’s deadline.

Same-day registration rules can vary significantly between states. In most cases, voters can register and cast their vote at the assigned polling station. But a handful of states, including Michigan and Montana, require them to go to a local election office to register. Alaska and Rhode Island only offer same-day registration for the presidential election. Anyone wanting to vote in state or local races must register early.

Voters who want to register on Election Day also typically need to bring documents that they wouldn’t need if they were just voting. Often this includes proof of residency, such as a utility bill, plus a valid ID – even in states that don’t have strict voter ID laws.