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Two things every guitarist should know about are ACTION and INTONATION.
Action is the heighth of the string from the fingerboard.
If the strings are close, we say the guitar has low action. If the strings are farther away, we call this high action.
Most players agree that lower action is easy to play on. But there are plenty of players that prefer the strings a little higher. (*Heavier string gauges require a higher action to give the bigger strings more room to vibrate).
You can set your preferred action on most any guitar. But first we need to make sure the neck is straight.
All wood necks require periodic straightening due to climactic change. Ever notice how sometimes a wooden door or window frame gets stuck? It’s usually just the wood expanding or contracting. The same climactic changes also affect your neck.
You might notice your action getting slightly higher or lower over time. Strings exert a constant pressure on the neck, and any seasonal change can upset this equilibrium.
Most modern guitar necks contain a device to counteract this change called a truss rod. The truss rod is actually two metal rods connected together on the inside of the neck.
On Fender-style guitars you can usually see the truss rod above the nut:
Or at the body end of the neck:
On Gibson-style guitars (& countless others) the truss rod is concealed behind a piece of plastic called the truss rod cover located on the headstock right above the nut:
When your neck isn’t straight, it affects the guitar’s intonation. Intonation is an instrument’s ability to play in tune with itself. *On a correctly intonated guitar, you should be able to play any open chord-then play the same shape 12 frets higher without sounding out of tune. It should just sound an octave higher.
When our guitar is in TUNE and our neck is straight we should try the chord/octave chord test I just spoke about*. Is it better? Worse? Don’t worry- we still have options…
This where the guitar saddles come into play. Eventually everyone noodling around on a guitar asks “Are these all supposed to be straight?”. The answer is usually NO. You see the individual saddles are used to “intonate” each string. If a guitar’s intonation is set, an open string and it’s 12th fret harmonic should read in tune with each other. When all of the strings (and their octaves) agree, the guitar is tuned and correctly intonated.