World leaders are seeking clarity from president-elect Donald Trump on a number of pressing issues, including trade and relations with NATO. While there’s a lot of guesswork about Trump’s policies, there’s one thing the rest of world should prepare for: a titanic shift in immigration.

Trump’s hard-line position on immigration doesn’t just affect undocumented immigrants; he wants to severely restrict legal migration too. Trump has vowed to “keep immigration levels, measured by population share, within historical norms.” Since 1970, the US’s foreign-born population has nearly tripled in percentage terms, to 13.3%, and it’s projected to reach 18% by 2065, according to the Pew Research Center.

Trump wants to make it much harder to get a green card (permanent resident status), but has yet to define what he means by “historical norms.” When the US began collecting data on the immigrant share of the total population in 1850, it stood at around 10%. It fluctuated before peaking at around 15% in 1890—due in large part to high levels of European immigration.

In 1921, the Emergency Quota Act began limiting immigration from a country to 3% per year of the number of people from that country who already lived in the US in 1910. The 1924 National Origins Act cut this quota to 2% and also excluded immigrants from Asia. Between the 1930s and 1970 the immigrant share dropped sharply, reaching a historic low of around 5% in 1970.

So if Trump’s ”historical norm” is from 1921 to 1965, the number of green cards issued each year would halve, according to the New York Times (paywall). If the norm runs from 1966 to 1976, when Trump was in his twenties, it would fall by 41%.

Whatever the target figure, Trump has announced a number of plans to make it harder to obtain a green card in the next four years. One of his most radical ideas is to stop issuing green cards altogether for a period of time, thus freezing the ability of existing US residents to bring family members to join them.