The compressor design point will be in an area of high efficiency whether the compressor is part of a gas turbine engine or whether it is used for pumping air into a blast furnace. However the compressor has to provide suitable performance at other operating conditions imposed on it which means a high efficiency is required over a wider range of operation. In the case of a gas turbine engine it must permit the engine to be started readily and accelerated rapidly to the design speed which means operating at lower speeds than the design speed. At speeds and flows away from the design point the compressor flow area reduction is not appropriate for the actual density rise. A NACA report illustrates pictorially the difference in contraction required at the design condition and at low speed. At lower speeds, for example, the gas hasn't been compressed enough to exit the compressor without adversely affecting the blade angles of attack through the compressor. Away from the design point the middle stages of an axial compressor continue to operate at about the optimum angle of attack but the front stages deviate one way, towards stall, and the rear stages in the opposite direction towards choke. The deviation was acceptable up to a design pressure ratio of about 5:1, producing only a loss in efficiency. At higher design pressure ratios prevention of rotating stall, which occurs at low corrected speeds, and choking was required with the introduction of inlet guide vanes which partially closed at low speeds, or by removing air part-way along the compressor at low speeds. Rows of variable stators or split compressors, which allowed the front stages to speed up and the rear to slow down relative to each other, would also be introduced for the same reason. When pressure ratios reached about 12:1 compressors would incorporate more than one of these features together.