What is intonation?
What is the best way to check my intonation and fix it if necessary?
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Intonation refers to the instrument being in tune along the fretboard.
An easy way to check the basic intonation of a guitar is to hit a 12th fret harmonic and compare the pitch with a note fretted at the 12th fret.
If the fretted note is sharp, the string needs to be lengthened.
If the fretted note is flat, the string needs to be shortened.
The length of the strings can usually be adjusted at the bridge of electric guitars and basses.
Tune each string to pitch before checking its intonation.
Intonation is all about notes being in tune along the fretboard. Frets are straight, so your guitar is pretty much always a little bit out of tune at any point down the fretboard. This is the reason you have compensation at the bridge and often the nut too. (Have a look at the bridge, the strings aren't all fixed at the same distance from the nut.)
Some companies have tried to address this by not having straight frets. I believe Steve Vai has messed around with this.
Intonation is pitch accuracy, whether it's a person's intonation or an instrument's intonation. When your guitar is tuned correctly, then it has the correct intonation. See here for ways to tune your guitar:
On a steel string acoustic you really have to trust that the manufacturers mathematics and building techniques have got the scale length correct. The frets in the correct location and the bridge saddle in the right place and at the correct angle. Then it can vary with string guage and the guitars set up. Now add the players techniquie and the word Compromise enters the equation..Don't always think that because a guitar has a brand name on it all of the above is correct. Electrics do offer Intonation adjustments ussually on every string or pairs of strings...Always check Scale length.. Measure from the Nut face to the crown of the 12th fret, then double it. Not all but a lot of Gibson guitars 24.75 inch scale length has crept to around 24.6 over the years and dependent on the factory. Your aim is to have your guitar when tuned to sound in tune and pleasant, whether playing open chords, bar chords with or without a capo.
I use a Korg Orchestral Tuner to test all the open strings and frets. Just for curiosity I took off all the frets and made small brass frets for every note on the guitar. I glued them on with Bostik Contact Glue. The frets are 3mm thick with a thin brass base about 1/4 inch square . Also just for curiosity they are all tuned to Kirnberger 111. And it works too .
I use a guitar tuner to do this, firstly make sure all your open strings are in tune, then play an open harmonic at the twelve fret and see if it's also in tune. The next part depends on the type of guitar you have. For a standard strat/super strat guitar you want to bring the saddle forward using the screw slightly then check the harmonic again, if it's flat you need to bring the saddle backward. There's a good youtube video here:
This isn't the ONLY reason your guitar could be intonated incorrectly though.
More info here:
My 2 cents : when you fret a note, you slightly change the vibrating length of the string; an harmonic at the 12th fret will be at the exact middle of the string, but the actual fretted note won't - think of a triangle formed by the open string, the vertical of the fret and the actual string being fretted.
When the fretted note goes flat, it goes flatter and flatter up the neck, and if sharp it goes sharper. If it's bad enough you can check it just by playing two notes an octave apart on the same string.
You can adjust the length of the string with a screw. On a fixed bridge, it's as simple as others have pointed out : check the high fretted notes with a tuner or the harmonics and fix.
With a floating bridge though, it's a bitch to setup, although depending on the model you have some very useful tools.
To fix intonation (on an electric guitar), you'll need to adjust the screws on the bridge:
This will lengthen or shorten the string enough to get the frets to sound in tune. More info
The main adjustment to correct intonation is making sure the 12th fret is in tune when the guitar is tuned correctly. That's the first step, but that's not all.
(Don't use the 12th fret harmonic for intonation. Use the open string and the 12th fret. Just don't.)
The second adjustment is more tricky. The Rule of 18 (actually 17.835) is that the next fret is placed 1/18th (or so -- see previous digression) of the way between the previous fret (or the nut) and the bridge, and this would be wonderful, except for fretting. This math does not take into account the thickness of the string. This means that, unless you adjust to account for it, you could be perfectly in tune on the low E with an open string and fretted at the 12th fret, the G at the third fret will be sharp. There are a few ways to handle this. First, develop a lighter touch and stop bending the string so sharp when fretting. Second, sometimes people tune and intonate so the low E and A strings are a little flat but the strings play in tune at the 3rd and 12th frets. This is also one of the issues that Feiten tuning is supposed to handle.
For many guitarists, intonation is something you think about when you change strings, and adjust with a screwdriver. Intonation is ultimately about your ears knowing what notes they should be going for, and your hands knowing how to get them. By bending strings, the best players can play perfect notes on a poorly-built and badly-intonated instrument. I'm not perfect in this, but embarrassing experiences with a lap steel showed me how bad I was, and I'm trying to learn to be better.