The Chicago blues is a form of blues music indigenous to Chicago, Illinois. Chicago blues is a type of urban blues. Urban blues evolved from classic blues as a result of the great depression and developed in the first half of the twentieth century during the Great Migration, when Black workers moved from the Southern United States into the industrial cities of the Northern United States such as Chicago. Chicago blues is based around the sound of the electric guitar and the harmonica, with the harmonica played through a PA system or guitar amplifier.