Scotland joined England in a political union in 1707. While Prime Minister Theresa May's centre-right Conservative Party has a majority in the UK Parliament, it has only one lawmaker in Scotland and 54 of the 59 Scottish seats are held by Scottish First Minister Nicola Sturgeon's Scottish National Party (SNP).
Legally, who decides?Britain's national Parliament is sovereign and is therefore the only legitimate authority to legislate on constitutional issues. Such matters are among the "reserved" topics that in general are not devolved to the Scottish Parliament.
With prior agreement between governments in London and Edinburgh, the UK Government can devolve power temporarily from Westminster to the Scottish Parliament, Holyrood, making a bill to legislate for an independence referendum in Edinburgh legally watertight. This is known as a Section 30 order.
This is what happened to allow the 2014 independence referendum.
Many observers believe May's government ultimately will give its blessing to a secession vote once more if requested, to avoid being seen to stand in the way of the Scottish people.