The resonance of a series RLC circuit occurs when the inductive and capacitive reactances are equal in magnitude. These reactances therefore cancel each other because they are 180 degrees apart in phase. The cancelling of the reactances of L and C causes the RLC circuit to behave like a simple resistor of the magnitude of its R value. The circuit impedance is lowest at the resonant frequency and hence the current flow through the device is maximized at resonance.