The frequency where oscillation occurs, defined by fs is called “series resonance frequency”. The crystal reactance curve shown here illustrates where mechanical resonance occurs. Series resonance occurs at the point the curve crosses zero. When the reactance of the motional inductance and shunt capacitance cancel, the crystal is at anti-resonance frequency, defined by fa. The frequency of a crystal operating at fa is inherently unstable. The region between fs and fa is called “area of parallel resonance” and is the region where most crystals are manufactured to operate.