The day after his administration unveiled a regulation that would allow it to indefinitely detain migrant families with children, President Donald Trump also revived talk of a much more radical step: abolishing automatic American citizenship for anyone born in the United States.

Trump said Wednesday that the rule, enshrined in the Constitution for more than 150 years and rooted in common law before that, was “frankly ridiculous,” and said the White House was “looking very, very seriously” at ending a policy that in the past he has called “a magnet for illegal immigration.”

Here is what you need to know about birthright citizenship and whether the president could put an end to it.

Does the Constitution guarantee birthright citizenship?

Yes. The 14th Amendment says, “All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.”

The part about jurisdiction creates a very narrow exception that today essentially applies only to children of accredited foreign diplomats. Other than that, the citizenship or immigration status of a person’s parents has not been held to have any affect on this right.

(The term “birthright citizenship” can also refer to another way a person can be born an American citizen: By having a citizen parent. This source of citizenship, created by federal law rather than directly by the Constitution, has not generally figured in the debate over illegal immigration, though it comes up sometimes in presidential elections.)

Can the president abolish it?

No. The president cannot amend the Constitution, and an executive order trying to end or restrict the right to citizenship of persons born in the United States would almost certainly be challenged in court as a violation of the 14th Amendment.

Trump has raised the abolition idea before, and some of his allies have suggested that the jurisdiction language in the amendment could be construed to exclude children of unauthorized immigrants. But the overwhelming consensus of legal experts is that such an argument would have no chance of prevailing in court.

Do other countries have the same policy?

Those in the Western Hemisphere generally do, according to a 2010 study, while those in the rest of the world generally do not, at least with regard to people who are not legal residents. Several countries that, like the United States, had traditions of universal birthright citizenship rooted in English common law, including Britain, Ireland, Australia, New Zealand and India, have restricted or abolished the right in recent decades.

What do critics mean by “birth tourism,” “chain migration” and “anchor babies?”

Those terms, used by anti-immigration hard-liners and other critics of birthright citizenship, tend to conflate two separate consequences of the policy.

Each year, thousands of pregnant women from other countries enter the United States on a valid visa, give birth to children who automatically gain American citizenship, and then take the baby back home or to another country. The practice, sometimes called “birth tourism,” is legal as long as the mother obtains her visa truthfully and complies with its terms.

Trump and his supporters, however, tend to talk instead about expectant mothers entering the country illegally to give birth to what they derisively call “anchor babies” who would give the family access to public benefits and a foothold toward legal residence.

Immigrants who have entered the country illegally have given birth to about 4 million children who are U.S. citizens, according to the Migration Policy Institute, a nonpartisan research outlet. But the overwhelming majority did not cross the border in utero. The Pew Research Center found that the parents of 9 out of 10 of those children entered the United States at least two years before giving birth, suggesting that the parents migrated for other reasons.

Whether they are reared in the United States or abroad, American children of noncitizen parents who reach the age of 21 can sponsor family members for legal permanent residency, just as any other American citizen can — a practice derided by critics as “chain migration.” Sponsorship by relatives has featured prominently in the stories of tens of millions of immigrants to the United States over the last century.