1921 Emergency Quota Act
This temporary act was the first bill to set a numerical limit on immigration. It established a 3% quota on incoming immigration based on the 1910 census – meaning that 3% of the population of that nationality documented in the U.S. at the time of the 1910 census would be allowed in per year as immigrants.
United States, Congress, House. 1921 Emergency Quota Law. Government Printing Office, 1921 [1]
As a result, the bill greatly affected southern and western European immigrants since those populations mainly arrived later in the 1900s.