A crystal resonator is a mechanically vibrating system that is linked, via the piezoelectric effect, to the electrical world. In the (simplified) equivalent circuit (of one mode of vibration) of a resonator, C0 is called the “shunt” capacitance. It is the capacitance due to the electrodes on the crystal plate (plus the stray capacitances due to the crystal enclosure). The R1, L1, C1 portion of the circuit is the "motional arm" which arises from the mechanical vibrations of the crystal. Some of the numerous advantages of a quartz crystal resonator over a tank circuit built from discrete R's, C's and L's are that the crystal is far stiffer and has a far higher Q than what could be built from normal discrete components.