The amount of amplification that an amplifier provides is called its gain. The gain is a ratio of the input signal level to the output signal level and is simply a number. Commonly expressed in dBindB, one should not express the voltage gain of an amplifier in dBindB unless the input and output impedance are matched as the gain of a typical amplifier is not related to its power output capability. For instance, if an amplifier has a voltage gain of 10, it might be said that it has a gain of 20dB because it actually would raise the power level of a signal by 20dB if the input and output impedance were matched. In practice, however, this is very seldom the case, and the true power gain is usually very much different from what would be predicted by the voltage gain.