Crystal oscillators are loosely divided with the frequency they work at and whether or not they are fundamental oscillators—get the job done in the frequency marked over the can or at even multiples of that (overtone oscillators). As you can see from your graph underneath, crystals have two resonant points. https://kingbookmark.com/story19018033/the-smart-trick-of-bulk-crystal-oscillator-that-nobody-is-discussing