Crystal oscillators are loosely divided because of the frequency they do the job at and whether they are fundamental oscillators—operate at the frequency marked to the can or at even multiples of that (overtone oscillators). As you'll be able to see with the graph below, crystals have two resonant factors. https://mypresspage.com/story4077325/the-single-best-strategy-to-use-for-bulk-crystal-oscillator