What Is "Intonation"?If you're reading this, hopefully you know what it means to have your guitar "tuned". If not, learning to tune your guitar is a pre-requisite for learning how to adjust your guitar's intonation. When we tune our guitar we're using an outside reference, often an electric tuner or perhaps a note on a piano (if you're my age you could even be using a tuning fork) to ensure that each of your open strings is vibrating at the correct frequency, producing the pitch that it needs to be. Intonation on the other hand, is a related yet separate issue. It is possible for your guitar to be perfectly in tune and your intonation can still be bad. Due to the fact that a great majority of guitars are constructed from wood, which is a substance that absorbs water, the neck of a guitar is liable to move slightly over time or due to a sudden change in humidity. Often this movement can be problematic because for your guitar to be in tune the entire way up the fretboard the 12th fret needs to be the mathematical halfway point between the bridge of your guitar (where the vibration of the string begins) and the nut (where the vibration ends). If the neck of your guitar has moved in such a way that the 12th fret is no longer perfectly the halfway point, your intonation is said to be bad and chords or melodies played on the upper part of the fretboard will sound out of tune, despite the fact that your open strings may be perfectly in tune.
What Tools Do You Need To Adjust Intonation?Thankfully, adjusting your intonation doesn't really require anything outlandish in the way of physical resources. In fact, there's a good chance that you already have access to these tools. For the sake of posterity, here's a list of some of the things you will likely need to use in order to adjust your intonation.
First of all, some sort of electronic tuner will be your best bet at ensuring that you do a proper job of adjusting your intonation. These three specific models are reliable standards, and ones that I have personal experience with so I'm sure you can use these to adjust your intonation effectively, but these are by no means the only electronic tuners that can get the job done.
Second, you will need either a phillips head screw driver, a smaller phillips head or perhaps flathead screwdriver, or an allen key depending on the specific model of your guitar. To determine which tool will work for your guitar, take a look at how the saddles on your bridge can be moved. Here are some examples:
All three of these guitars will require a different tool to move the saddles. The Stratocaster on the far left will need a phillips head screwdriver. Due to the easy accessibility of the screw heads, it is likely that just about any phillips head screw driver will do. The SG in the middle image will need a flat head screw driver. These screw heads are less accessible because of the design of the bridge, so it might be a good idea to use a precision screw driver like the kind used on eyeglasses. The far right image is a wrap tail bridge, and these bridges do not have individual saddles. These style of bridges need an allen key to move the bridge at either side.
How To Adjust Your IntonationThe first thing to do when adjusting your intonation is to ensure that all of your open strings are as in tune as they can possibly be. Next, you must go string by string and compare either the open string or the 12th fret harmonic with the fretted note at the 12th fret. If your open string (or 12th fret harmonic) is in tune, and the fretted note at the 12th fret of that same string is also in tune then your intonation is fine. If when your open string is in tune the fretted note at the 12th fret is more than a couple of cents sharp or flat, your intonation is out and you need to adjust it.
As stated previously, the 12th fret must be the halfway point of the string's vibration. If the open string is in tune but the fretted note at the 12th fret is sharp, what this means is that the distance from the 12th fret to the bridge saddle is too short. In order to correct this, you will need to move the saddle back by using either an allen key or a screw driver. Once you have moved the saddle back a little bit, re-tune your open string (moving the saddle will put the open string out of tune). Now, with your open string in tune check the tuning again of the fretted note on the 12th fret. If it is not completely corrected, moving the saddle back must have at least improved it a little bit. Repeat this process until both the open string AND the fretted note at the 12th fret are in tune together.
If when your open string is in tune, the fretted note on your 12th fret is flat, this means that the distance between your 12th fret and your saddle is too long. You must correct this by moving the saddle forwards until both the open string AND the fretted note at the 12th fret are in tune using the same process outlined in the previous paragraph.
Many electric guitars are constructed with bridges that allow you to adjust the intonation of a single string without affecting the intonation of any other string. Stratocasters, modern Telecasters, most Les Pauls and 335's all come with these styles of bridges. Some notable popular exceptions to this are Telecasters with only 3 saddles, and any guitar with a wrap tail bridge. On these types of guitars, adjusting the intonation on one string will always affect the intonation of another string. This can make for a frustrating experience if you are new to it, but patience here is key.
I should mention as well that most acoustic guitars do not have moveable bridge saddles at all. On an acoustic guitar, the intonation is adjusted by use of the truss rod. If you are new to the maintenance of your guitar and you haven't messed with a truss rod before, I highly recommend taking your acoustic guitar in to a tech to adjust any intonation issues.
Thank you for checking out my thoughts on how to adjust and correct your intonation! I'm honoured to be a small part of your journey with the guitar, and it is a privilege that I never take for granted :)