http://www.travel.state.gov/pdf/Immigrant%20Visa%20Control%20System_operation%20of.pdf
Foreign State Chargeability:
Ordinarily, an immigrant is chargeable for visa purposes to the numerical limitation for the
foreign state or dependent area in which the immigrant's place of birth is located. Exceptions are
provided for a child (unmarried and under 21 years of age) or spouse accompanying or following
to join a principal to prevent the separation of family members