Color-color diagram of Cyg X-1. The diagram shows distinct regions where the source undergoes several turns. Numbered gray arrows indicate preferred tracks which Cyg X-1 follows when transitioning between the hard and the soft state. Some observations in the hard state show “nose”-shaped tracks where the color-color bends down towards a harder spectral shape (“1.”), which is called dipping. When Cyg X-1 softens, the source shows large variations dominantly in soft color (“2.”). As the source transitions through the intermediate into the soft state, it follows a distinct zig-zag shape driven by the soft color (“3.”). When Cyg X-1 is in the pure soft state, it still ranges significantly in hard color (“4.”).