Crystal oscillators are loosely divided because of the frequency they perform at and whether or not they are fundamental oscillators—operate within the frequency marked over the can or at even multiples of that (overtone oscillators). As it is possible to see within the graph below, crystals have two resonant points. https://bookmarkfriend.com/story18971447/how-bulk-crystal-oscillator-can-save-you-time-stress-and-money