Because of the nature of alternating currents and voltages, the voltages are different at different positions on the sine wave: Position A = positive peak value, Position B = RMS (effective) value, Position C = average value, Position D = negative peak value.
RMS (root means square) is also called the “effective” voltage. It is equal to 70.7% of the peak-to-peak value of the sine wave. This is also called the equivalent value in a circuit that contains direct current only. It is the value that most voltmeters and ammeters read.