What is the difference between series resonant and parallel resonant crystals?

All quartz crystal resonators have a series resonant frequency (fs, frequency of lowest impedance). At this frequency, the crystal appears resistive in the circuit. Crystals can be "pulled" from this series frequency by adding reactance (capacitance) in series with the crystal. When operated in combination with an external load capacitance (CL) the crystal oscillates in a frequency range slightly above its series resonant frequency. This is the parallel (load resonant) frequency. When ordering a parallel crystal, always specify the nominal parallel resonant frequency and the amount of load capacitance in picoFarads (pF). Alternatively, a standard value of CL (such as 20pF) can be ordered; the capacitor values are then calculated to match the crystal CL. Note that a series type will oscillate in a parallel oscillator, and vice versa. The observable difference is a frequency offset of less than + 1000PPM (0.1%) from the nominal frequency. 4.  more