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Crystals must be custom tuned by cutting or grinding to exact dimensions and tolerances to achieve a highly accurate desired frequency.Crystals age over time and cause the frequency to drift from the original design. Because a crystal can operate at either its series or parallel resonance frequencies a crystal oscillator circuit needs to be tuned to one or the other frequency as you cannot use both together.A quartz crystal can act as either a capacitor, an inductor, a series resonance circuit or as a parallel resonance circuit.==identification==* [http://www.electronics-tutorials.ws/oscillator/crystal.html Quartz Crystal Oscillators]