I once tuned a piano with a harmonica and a guitar pick and it came out pretty good. (And no I didn't us a crescent wrench.) But, out of boredom thought I'd at least inquire as how to do it right and came across this site:
http://piano.detwiler.us/
Anyway,could someone explain why it's the A key?
Technical Sidenote: Ideally, you should just tune the middle octave with the tuner, then tune all the higher and lower registers by ear. In fact, a purist professional tuner may just tune the "A" with a fork and tune the rest by ear! It is difficult to match precisely the electronic tuner consistently. Also note that the only truly universal note all pianos share is A (above middle C) equals 440 Hz. From there a professional tuner essentially tunes the piano to itself (though he may use electronic aids and software to calculate the precise frequencies, such as those from TuneLab) because the vibration rate for other notes, particularly on the extreme octaves, may be in tune outside of the expected mathematical progression. This is so because of the differences in structure and materials among pianos. However, I went with the electronic tuner (for the first string in each note) whenever I could and it turned out OK.