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The concept of equivalent characteristic parameters of a liquid crystal (LC) cell is introduced together with the characteristic circle and the characteristic ellipse. They are all related to the twist angle and retardation of the LC cell. Such equivalent characteristic parameters can provide a framework for analyzing LC modes, as well as providing a means to measure LC cell parameters. A new Stokes parameter method is also introduced to measure these characteristic parameters. Thus the twist angle and retardation of the LC cell can be determined accurately.