For circuit operation, the variations between de and ac functions are not too dissimilar. For de signal voltages, the individual stages of the instrument are directly coupled. Direct coupling can also be used for ac voltage signals, but typically capacitors are used to pass only the ac signal voltages from one stage to another. The test indications for ac must be handled slightly differently because of the nature of ac waveforms and the different ways of describing their levels. One ac wave has average, effective (RMS), and peak values. The analog meter movement responds to the average ac value, but we all use the effective or RMS value to describe a waveform, such as 120-V ac line power. The ac meter is calibrated for the effective value, even though it reacts to the average value.