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There are three ways to measure voltage in an AC circuit:
Peak voltage: Ameasurement of thelargest voltage.
Peak-to-peak voltage: The difference between the highest and lowest peaks of the voltage reading.
DCAClab calculates RMS by sampling the input voltage and doing the sum-of-the-squares, simulating the high quality voltmeters.
Notice the dashed orange line in the oscolicope, its the RMS value of voltage. A good article that explains the math behind RMS can be found here.