quartz tuning fork and its series and parallel resonances
Your plots are not accounting for the impedances in circuit, you are just viewing it from the series resistance perspective. From the oscillators in circuit perspective parallel resonance is sharp.

Series resonance is not commonly used, because it requires an inductor in the oscillator circuit to cancel out the electrode capacitance of the crystal.

The simplest and cheapest oscillator circuits don't use an inductor, they use capacitors, and thus must be in parallel resonance. (the crystal is operated in the inductive region)

Series resonance is commonly used for overtone oscillators, as without an LC filter, the oscillator will usually prefer the fundamental.

Note that manufacturers can't/don't "make" the crystal for either resonance. All they can do is tune the crystal to frequency at a specified load capacitance. The crystal is identical, and oscillates the same. (just off frequency of you use different load C.)



In series resonance signal passes through the crystal. In parallel resonance, it is blocked. Which you see depends on how your test circuit is arranged.







simulate this circuit &ndash; Schematic created using CircuitLab