The economic reforms of President Carlos Salinas de Gortari had a tremendous effect on the city, as a number of businesses, including banks and airlines, were privatized. He also signed the North American Free Trade Agreement (NAFTA). This led to decentralization and a shift in Mexico City's economic base, from manufacturing to services, as most factories moved away to either the State of Mexico, or more commonly to the <answer>northern border</answer>. By contrast, corporate office buildings set their base in the city.
Where did the factories of Mexico City move to most commonly?
northern border