Crystal oscillators are loosely divided with the frequency they perform at and whether or not they are basic oscillators—operate at the frequency marked on the can or at even multiples of that (overtone oscillators). As you'll be able to see with the graph down below, crystals have two resonant points. https://privatebookmark.com/story18981517/helping-the-others-realize-the-advantages-of-bulk-crystal-oscillator