Crystal oscillators are loosely divided because of the frequency they function at and whether or not they are basic oscillators—do the job with the frequency marked on the can or at even multiples of that (overtone oscillators). As you could see in the graph under, crystals have two resonant details. https://thesocialdelight.com/story4452195/the-single-best-strategy-to-use-for-bulk-crystal-oscillator