Why Quartz crystals cut for parallel resonance are ground to hit their marked frequency only when they see a specified load capacitance across their terminals, typically something like 18, 20 or 32 pF. The circuit capacitance combines with the crystal's motional and holder capacitance to set the exact resonant point, so the oscillator must present that load value. Too little or too much load capacitance pulls the crystal slightly high or low in frequency, which is why many oscillators include a small trimmer for netting.