Why A quartz crystal's mechanical resonance is modeled electrically by a motional arm of series L, C and R, where L represents the vibrating mass, C the mechanical stiffness, and R the mechanical losses. Across that arm sits a separate shunt capacitance C0 formed by the metal electrodes plated on the quartz plus holder and stray capacitance. This Butterworth-Van Dyke model is why a crystal has two closely spaced resonances: a series resonance from the motional L and C, and a slightly higher parallel (antiresonant) point where the motional arm looks inductive and resonates with the shunt C0.
Watch out The choices that lump the electrode and stray capacitance together with the motional capacitance into one RLC branch miss the point: the shunt capacitance must be a separate parallel element, since it is what creates the second, parallel resonance and the gap between the two resonant frequencies.
Crystal = series RLC motional arm with the electrode C hung across it. Two Cs, two resonances.