Years ago, before the days of electronic tuners (or even digital watches or pocket calculators) I tried to buy a tuning fork, but I made the mistake of telling the salesman in the shop that it was to tune a guitar. He flatly refused to sell me one, insisting that I needed a six-note plastic thing to blow into, which was useless as the sound was too different and I couln't tell if the note was right or not, whereas the vibrations from a fork and a string were easy to compare, and once you have one string tuned you can tune the rest to that one.