Because the migrants concentrated in the big cities of the north and west, their influence was magnified in those places. Cities that had been virtually all white at the start of the century became centers of black culture and politics by mid-century. Residential segregation and redlining led to concentrations of blacks in certain areas. The northern "Black metropolises" developed an important infrastructure of newspapers, businesses, jazz clubs, churches, and political organizations that provided the staging ground for new forms of racial politics and new forms of black culture.