Q: What happened Thursday?

A: A referendum was held Thursday to decide whether the United Kingdom should leave or remain in the European Union. Leave won by 52 percent to 48 percent. Turnout was 71.8 percent, with more than 30 million people voting.

Q: How did different parts of the United Kingdom vote?

A: England voted strongly to leave the E.U., by 53 percent to 47 percent, as did Wales. Scotland and Northern Ireland both backed staying in the E.U.

Q: What is the E.U.?

A: It's an economic and political partnership of 28 European countries. It was formed after World War II to generate economic cooperation. It has since grown into a "single market," allowing goods and people to move freely among the countries. It has its own currency, the euro, which is used by 19 member countries.

Q: Now what?

A: To leave the E.U., an agreement called Article 50 of the Lisbon Treaty must be invoked. Terms of the country's withdrawal will then be negotiated.

Q: Could Parliament block the exit?

A: The referendum result is not legally binding. Parliament still has to pass laws that will get Britain out of the bloc, then both chambers must ratify the departure agreement.

Q: Can the vote be reversed or retaken?

A: More than 2 million Britons have signed petitions asking for a second vote, but that seems unlikely.

Q: Is this the first time a country has left the E.U.?

A: Yes, but Greenland, a territory of Denmark, voted in 1982 to leave — which it did.

Q: Why did British Prime Minister David Cameron resign after the vote?

A: He led the effort to remain in the E.U. and the defeat undermines his leadership. He said he wants a successor who agrees with the "leave" decision to handle the divorce.

Q: How will the United States be affected?

A: The stock market tumble on Friday was a reflexive reaction, but a climate of economic uncertainty could continue to hurt U.S. stocks. The E.U. could further unravel, creating political instability across the globe, and U.S. corporations that do business across Europe could decide to leave Britain.

Q: How will trade and defense be affected?

A: Britain has the bloc's second-largest economy after Germany, and now it must come up with new trade agreements. Britain remains a leader of NATO, the European defense coalition, and is a permanent member of the United Nations Security Council.

Q: How did President Obama and other American politicians react?

A: Obama said that "the people of the United Kingdom have spoken, and we respect their decision." He promised that the E.U. and Britain would remain "indispensable partners of the United States." Democratic presidential candidate Hillary Clinton said "our first task" is to make sure the economic uncertainty "does not hurt working families here in America." Republican presidential candidate Donald Trump said, "I said this was going to happen, and I think that's a great thing."

News services