Automatic tuners make getting an off-key guitar back into tune much simpler, but these devices are not the only way of accomplishing the task. The guitarist who has not acquired and automatic tuner can use one of several other instruments or devices plus a little music theory to do the job quite accurately.
If an in tune piano or keyboard is handy, it provides the simplest way to find the notes for tuning the guitar. This, however, does require the player to know where the notes fall on the keyboard to be able to match the pitches properly. Starting with the thickest string of the guitar, which will produce the lowest note, play the low bass E on the piano. Turn the tuning knob of the guitar corresponding to the thickest string while plucking the string until the pitches match. From here, the other strings can be tuned in the same way, matching each pitch to the corresponding one on the keyboard Tune the second string to the B above the first E, then the third to G on the top space of the bass staff, fourth string to D immediately above Middle C, fifth string to A on the second space of the treble, and the final string to the E four octaves above the first string.
A similar but more portable technique involves the use of a pitch pipe, where each pipe has been tuned to one of these notes, and as the guitarist blows each pipe, he or she can match the pitch on the pipe to that of a string on the guitar. Since the pitch pipe does not go out of tune as a piano would, many player argue that using such a device makes tuning the guitar very simple.
A computer with a good music composition program combined with the guitarist knowing how to read the notes can also simplify the tuning process when the computer is handy. Once the guitarist enters the notes, preferably individually, for how the guitar should be tuned, the chord can be saved and retrieved almost instantly when the guitar needs to be tuned. Each of the notes, E, B, G, D, A and high E should be entered into the program in its own measure so the computer will hold the note as the guitarist uses it to tune the string by matching the pitch. Learning to tune a guitar in this way gives the guitarist another resource for tuning which will not lose its pitch in adverse climate conditions.
Of course, any of these devices and methods can be combined with the tried and true method of matching the low E string's pitch to that of the device supplying it. Once that pitch has been matched, the guitarist will often hold the E string down at the fifth fret and match the B string's pitch to it. From there, hold the B string at the fifth fret and match the pitch of the G string to it, and do the same to the G string to get the pitch for the D string. However, be careful tuning the A string and fret the D string at the fourth fret instead to get the proper pitch, as A is only a major fourth from D, not the typical interval of a fifth as found between the other strings. To get the high E for the sixth string, return to fretting the A string on the fifth fret after tuning it to produce the tone for E and listen for the strings to sound in unison. This method works nicely with electronic tuners as well, since many of them produce only the low E frequency for a guitarist to use in tuning his or her instrument.