Many (many, many) moons ago, when I was in college, I had the bright idea to design a digital tuner for my bass guitar. Pretty cool stuff for the time (1981/1982), but never quite completed. I don't even remember the details, but I do remember that one of my biggest challenges was to define what the "correct " frequencies were. And it turned out to be a pretty daunting task.
Of course, nowadays, with the internet (and Google!), it's just a matter of a 0.26 secong search, and then picking from the list. THIS would've made my life so much easier back then.