I busted a string on Sunday, and proceeded to do the fastest string change on an acoustic that I've ever done. I had clippers, capo and strings, but what I did not have was a portable tuner. The only one I had along was integrated into my multi-effects unit.
Tuning a guitar to itself is doable. Fifth fret to the next string's open. Fifth fret harmonic to the next string's seventh fret harmonics. 12th fret to the next string's seventh fret. Counting the beats and all. All that is a good to practice, to work on, to know. Magic boxes are good to have, because pitch pipes are horrible for tuning to, but you should still know how to tune your guitar to itself.
But that's one thing. The trick, the thing to learn, is a way to remember what a concert-pitch. And I thought I had that trick. "The Theme to Peter Gunn". One of the first things I ever learned on guitar. I get that riff going, and it sounds right, I thought, then I know I have the low E right. I honestly thought I had it.
Until I got back to the tuner.
I had it tuned to C. Not E. 4 half-steps flat.
Back to the drawing board.