To provide a correct reading of the difference between the frequency and phase angle between two generators or a generator and the network, when connected in parallel. When the difference is zero, the instrument's needle does not move from the synchronization mark located in the centre of the scale. The instrument scale is divided in two areas marked with the (+) and (-) signs. These signs indicate whether the machine being connected has a higher or lower frequency that the other one, respectively. Synchronism is achieved when the needle is on the (-) side, slowly turning towards (+). The needle of the instrument starts to turn in the correct direction when the difference in frequencies is 1.5 Hz for three-phase systems or 0.5 Hz for single-phase systems.