This fork would probably still work, but you get the idea.
Note: As will be shown below, this method of tuning, however common, is inherently flawed. Still, it is probably worth a brief study for the sake of misguided tradition and culture.
When tuning the guitar using harmonics, I start with an A = 440 Hz tuning fork. I choose this fork because A = 440 is the internationally standardized tuning reference tone. In standard tuning, when you play the harmonic on the fifth fret of the fifth string, you should create a 440 Hz tone. So, to tune the fifth string (A), I first strike the fork on the bony part of my knee. Then, I quickly pluck the harmonic on the fifth fret of the fifth string. Next, I place the ball of the tuning fork on the bridge of the guitar (find the sweet spot for it on your bridge). Finally, I use my fifth string tuner to match the tone of the tuning fork (which is now resonating through the guitar). As you approach being in tune, you will hear the warble of the tones becoming longer and longer in duration. This is what I call “smoothing out” the tone. When the warbles are as long in duration as you can possibly get them (theoretically having disappeared), you have tuned your string.
You will want to start with a fifth string that is flat, and then tune up until it is in tune with the reference tone of A = 440 Hz. In fact, when using tuning hardware to tune a string, whether tuning to a fork or another string, always tune by tightening a flat string; never by loosening a sharp string. (You always hear musicians say “Get tuned up and we’ll get started.” You never hear them say “Get tuned down.”) So, for example, when I am tuning to the tuning fork, I will make sure that the fifth string is quite flat before starting. I do this because I want the hardware “nice and tight” by the time I have gotten “tuned up.” This helps prevent string slippage and better maintains the tuning. Okay, now let’s move on to tuning string-to-string.
The next string I tune is the sixth string. Now that we have tuned the fifth string to the fork, we may tune the sixth string to the fifth string. To do this, I will pluck the harmonic at the seventh fret of the fifth string (this is E at 330 Hz, since we have, in effect, already tuned our open fifth string (A) to 110 Hz) and tune the harmonic at the fifth fret of the sixth string to match that pitch. Now, if you see this as problematic, congratulations, you’re correct. In standard, equal temperament tuning, E = 329.62 Hz. So, if you match the harmonic of the seventh fret on the sixth string to the harmonic of the fifth fret on the fifth string, you have succeeded in improperly tuning the sixth string. We now have effectively tuned our open sixth string (E) to 82.50 Hz, when the correct standard is E = 82.41 Hz.
Let’s push on with this method, so that we will better see why it’s probably best not to use it.
Now that we’ve tuned the fifth and sixth strings, we may move on to the fourth string. Here, I will tune the harmonic on the seventh fret of the fourth string to the harmonic on the fifth fret of the fourth string. By doing this, we are bringing the harmonic on the seventh fret of the fourth string up to A = 440 Hz. This means that – without showing the math involved, not because it’s secret, but because it’s boring – we have effectively brought the pitch frequency of our open fourth string (D) to 146.67 Hz. The tempered standard for D? 146.83 Hz. Success. We have failed again.
Now let’s use our freshly out-of-tune fourth string as a reference for tuning the third string, and compound the problem. At this juncture, our reference tone will be our fresh D = 293.34 Hz as voiced by plucking the fifth fret on the third string. We will now tune the tone of the harmonic on the seventh fret of the third string up to our reference tone. Once accomplished, this effectively means that our open third string (G) is now at 195.56 Hz. The correct frequency? G = 196.00 Hz.
There are two more strings left to corrupt. For these, we will use the harmonic on the seventh fret of the sixth string (B as 247.50 Hz) as our reference. We will play the open second string (B) and tune accordingly. The correct standard for the open second string (B) is B = 246.94 Hz.
Finally, let’s jack up the first string (E). We will use the seventh fret on fifth string (E as 330 Hz) and tune the open first string (E) accordingly. The correct standard for the open first string (E) is E = 329.62 Hz.
While the fingers-free voicing and the purity of the harmonic tones might make this method of tuning attractive, it is not apt. The human ear can hear very slight deviations of pitch, and the discrepancies above are significant. We would be wise to choose a different tuning method.