The Blues form, ubiquitous in jazz, rhythm and blues, and rock and roll is characterized by specific chord progressions, of which the twelve-bar blues chord progression is the most common. The blues genre is based on the blues form but possesses other characteristics such as specific lyrics, bass lines and instruments. Blues can be subdivided into several subgenres ranging from country to urban blues that were more or less popular during different periods of the 20th century. Best known are the Delta, Piedmont, Jump and Chicago blues styles. World War II marked the transition from acoustic to electric blues and the progressive opening of blues music to a wider audience, especially white listeners. In the 1960s and 1970s, a hybrid form called blues-rock evolved.