The tuner has a large range from E0 up to C9, a very high accuracy of less than 0.05 Hz (waves of longer than 20 seconds) and can measure to three reeds at the same time. Because of this the tuning of the accordion can be measured without disabling reeds before opening the case. The real frequencies of the reeds which are influenced by each other and by the case can be measured now. This way it is considerably simpler and quicker to select the reed that deviates and needs attention. The tuner measures beatings that arise because the reeds sound together and shows these in Cent or Hertz. Beatings cannot be measured accurately enough by measuring the reeds separately, but by measuring them at the same time it is accurate enough. The measured beatings are compared to the beating list of the accordion concerned which indicates the desired beating for each note. This beating list can be produced rapidly and simply using a graph in the tuner.