Voters in Scotland began casting their ballots in a historic referendum on whether to go it alone as an independent country or remain part of the U.K. Photo: Getty Images

Officials say they are expecting the largest turnout for any Scottish vote on record. In total, 4.29 million people having registered to vote, the biggest electorate Scotland has had. Recent polls suggest more than 90% of voters plan to turn out Thursday. In a similar vote in Canada in 1995 on whether Quebec should secede, 94% of registered voters cast their ballot.

In Scotland, everyone the age of 16 or over is eligible to vote on Thursday, even though the age requirement is usually 18 years in general elections. Alongside Scots, British citizens and those from the European Union and Commonwealth countries who live permanently in Scotland will also have a vote.

How the Vote Works

  • Polls open at 7 a.m. local time and close at 10 p.m.
  • The ballot has one question: "Should Scotland be an independent country?"
  • Officials in the 32 electoral districts will count the votes and report their totals to the chief counting officer in Edinburgh.
  • The outcome will be decided by a simple majority.
  • Officials expect to announce the result at around 6 a.m. or 7 a.m. local time but warn it could be later.

People born in Scotland but who live elsewhere don't have a vote.

The outcome will be decided by a simple majority, which is the same method used in the Quebec referendums in 1980 and 1995.

A referendum on devolving power to Scotland from London in 1979 gave victory to Scottish nationalists seeking greater autonomy, but the result was ruled null and void because not enough people turned out. It took until 1999 before Scotland's parliament, which was dissolved in 1707, was re-established at Holyrood in Edinburgh. There are no such turnout thresholds on Thursday's ballot. (See what an independent Scotland would look like.)

There are nearly 6,000 polling stations across Scotland, from the Shetland Islands off the northeastern coast to Dumfries and Galloway on the southern border with England.

Once the polls close, officials in the 32 electoral districts will count the number of votes and report their totals to the chief counting officer in Edinburgh. The officer will then publicly announce the overall national result early Friday morning. Officials say they expect this at around 7 a.m. local time (2 a.m. EDT), but warn it could be later.

As the regional totals come in, among the key ones to watch for are the major cities of Glasgow, Edinburgh and Aberdeen, which will be the last three districts to declare their votes. Polls suggest these cities are almost as divided as the country as a whole. Of the three, genteel Edinburgh appears least likely to vote 'yes,' according to a recent poll by ICM which put support for independence in Scotland's capital and the surrounding area at only 38%. Support for independence is higher in Glasgow, Scotland's biggest city and its trendiest, but even there the 'no' vote still appears slightly ahead. Aberdeen, the nation's oil hub, is also split.

Pollsters caution that the margin of error on their regional samples is even bigger than their nationwide surveys.

In the U.K. during elections, broadcasters typically commission private exit polls, yet none of the main networks have said they have any plans for one.

If Scotland votes for independence, the Scottish National Party has said it would like Scotland to formally secede from the U.K. by March 2016. In the interim, the Scottish government and politicians in Westminster, where the U.K. government sits, would embark on what is expected would be bitter haggling over key issues, such as how to divide valuable oil reserves and defense assets. They would also have to determine what currency an independent Scotland would use. Some Scottish companies have already said they would move their headquarters and legal operations to England if Scotland becomes independent to protect themselves from financial uncertainties including tax.

If Scotland votes to remain part of the union, fraught discussions are still likely to ensue.

Prime Minister David Cameron and senior politicians from the U.K.'s main political parties have pledged to immediately begin the process of legislating the further transfer of powers to the Scottish government from Westminster. But with disagreement between the leaders of the main political parties over which powers to devolve, discussions are expected to be protracted.

Write to Jason Douglas at jason.douglas@wsj.com