Green cards are immigrant VISAS giving foreigners permanent-resident status in the United States. Unlike visas, which are granted for a specific length of time to engage in specific business activities, green cards allow non-U.S. citizens to reside in the country indefinitely. Green-card status also allows individuals to become U.S. citizens after five years (three years if the immigrant acquired the green card through marriage to a citizen). Most green cards, named such because of their color, are allocated based on the relationship of the applicant to U.S. citizens, but some are available based on business criteria. Employment-based green cards called First Preference Petition are issued to foreigners with extraordinary ability, including outstanding professors and researchers and certain executives and managers of MULTINATIONAL CORPORATIONS. Second Preference Petition cards are issued to members of professions holding advanced degrees and people of exceptional ability in the sciences, arts, and business. Third Preference Petition cards are available to skilled workers, professionals, and other workers, while Fourth Preference Petition cards are for special immigrants, including religious workers. Finally, Fifth Preference Petitions, known as million-dollar green cards, are available for people actively investing in a new business that will create at least 10 new full-time jobs for U.S. workers. The million-dollar requirement is reduced to half that amount for investment in low-population or high-unemployment areas of the country. Green cards are difficult to obtain and often require hiring specialized legal assistance. Foreigners in the United States on visas are sometimes subject to different rules when applying for permanent-resident status. In the 1990s the United States created a lottery system for 50,000 green cards annually.