What Guitar Tuners Measure
The pitch and frequency of sound are related. The higher the frequency, the higher the pitch. A guitar tuner can't "hear" pitch, but it can detect frequency. The frequency of a wave is the number of wave peaks that pass a fixed point in a given amount of time. Frequency is measured in Hertz (Hz), which corresponds to the number of crests passing in a single second. The range of frequencies audible to the human ear ranges from around 8 to 14,000 Hz.
Reading a Guitar Tuner
Notes are fixed at particular frequencies. For example, the A above middle C is always 440 Hz. Middle C is about 261.626 Hz and the E above it is at about 329.6 Hz. All other notes, including sharps and flats, have their own fixed frequency. A guitar tuner identifies the frequency of an incoming signal and records it in relation to these fixed standards. If the signal frequency matches the standard exactly, the tuner reflects that the note is in tune. If the frequency is a little too high or low relative to a standard frequency, the tuner will show the note as being a little sharp or flat. The display on most tuners is a series of LEDs lined up in a row with the center light representing in tune. A note that is only slightly flat will only be a few lights away from center, where a very flat note will be farther away--or even show up in relation to lower standard frequency. Other tuners only use one light for sharp, flat and in-tune notes; a digital display shows how near or far the note is from the standard pitch.
Using a Tuner
While most tuning devices have microphones that pick up the notes of acoustic guitars and other instruments, an unplugged electric guitar is not loud enough to be read by these devices. If you're tuning an electric guitar, connect the instrument to the tuner with a standard 1/4-inch guitar cable. To tune the guitar, play an open note on a single string and let it ring until it registers on the tuner. Determine if the note is flat, sharp or in tune. Tighten or loosen the string at the tuning peg, and play it again, repeating the process until the note is perfectly in tune.