The compressor has operating boundaries at the flow extremes for a particular speed which are caused by different phenomena. The steepness of the high flow part of a constant speed line is due to the effects of compressibility. The position of the other end of the line is located by blade or passage flow separation. There is a well-defined, low-flow boundary marked on the map as a stall or surge line, at which blade stall occurs due to positive incidence separation. Not marked as such on maps for turbochargers and gas turbine engines is a more gradually approached, high-flow boundary at which passages choke when the gas velocity reaches the speed of sound. This boundary is identified for industrial compressors as overload, choke, sonic or stonewall. The approach to this flow limit is indicated by the speed lines becoming more vertical. Other areas of the map are regions where fluctuating vane stalling may interact with blade structural modes leading to failure, ie rotating stall causing metal fatigue.