One interesting bit of trivia (I always wondered about but finally read-up on): Early on, amateur radio bands were allocated in a manner that ensured (in all but one case) harmonics would bleed into another, higher amateur band, rather than into a frequency used for another purpose. Changes in the late 1920’s dispensed with this[1], but we do still see rough multiples in many cases (160/80/40/20/10 meter, 60/30/15 meter, etc.).