Reality Check: How much child benefit goes overseas?
Obscure rules can cause heated arguments.
EC regulation 883/2004 falls into that category.
In bland terms, it sets out the laws that protect social security rights when people move within the European Union.
In practice, it allows EU nationals living in the UK to claim child benefit and have it paid in their home nation, to the fury of many Britons.
Similar rights apply for Britons living in EU countries.
The principle is probably more infuriating to some than the cost, because in reality the numbers taking advantage of the law are comparatively quite small.
According to data from Revenue & Customs published in March, in August 2016 there were 7,400,000 families in receipt of child benefit in the UK, which meant that nearly 13 million children (12,880,000) were getting the benefit.
Of that total, almost 21,000 families are classed as being either "foreign or not known", looking after almost 33,000 children.
That means that 0.3% of families, and 0.26% of all children who receive child benefit are not UK based.
And the numbers are falling.
Those figures from August 2016 represent a 20% decrease in the number of families claiming child benefit, and a 19% reduction in the children receiving the payment.
Not all of that category are EU nationals, of course.
For that data, we need to go a bit further back.
According to a paper published by the House of Commons Library in July 2014, there were just over 34,000 children of EU nationals receiving child benefit but living abroad in December 2013.
Families receive £20.70 a week for the first or only child and then £13.70 for each subsequent child.
Going back to the analysis published in March, the total paid to the almost 33,000 foreign national children would work out at approximately £33.5m for 2016.
Migration Watch, which pushes for lower immigration to the UK, did a similar calculation in 2013 (when the number of children was higher but the benefit rates were lower) and found the cost then to be £36.6m.
To put that figure in context, child benefit in total cost the Exchequer about £11.3bn in 2015-16.
Reducing the amount of child benefit that EU nationals could claim for children living outside the UK was one of David Cameron's key demands of fellow EU leaders in the run-up to last year's Brexit referendum.
He secured an agreement to reduce the weekly total they could claim, to better reflect the cost of living in each country - the British argument was that child benefit was far more generous than many foreign nationals could receive in their own country.
But, following the referendum result, that agreement has been withdrawn, and how much child benefit - if any - EU nationals can claim after Brexit will form part of the upcoming negotiations.
One final point - it's not just the British who want to see these bills reduced.
The German government, for instance - which pays far more in child support than the UK does, even to foreign nationals - is also pushing for reductions in the payments, again to better reflect the cost of living in the country in which the child lives.