In the beginning of the 20th century, many new things were built in the city, like skyscrapers and the New York City Subway. The first subway transit center, the Interborough Rapid Transit or "IRT", opened to the public in 1904. The installation of the Subway helped tie the new city together, as did new bridges to Brooklyn. In the 1920s, many African-Americans came to live in Manhattan during the Great Migration from the American South, and the Harlem Renaissance. New York City became the most populous city (city with the most people) in the world in 1925, overtaking London, which had held the title for a century.