Keep those strings out of your way when you work on your guitar. It’s easy (easier) with string spreaders.
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Over the next little while I'll give you some information on how to assess and set the action on a number of guitars and basses. If every, single instrument isn't covered, there should be something that's similar enough to an instrument like yours, or—at least—enough information to figure out how to approach your own.
Before we begin, you should remember that intonation is the last thing you will do when setting up your guitar or bass. If the nut, action, relief and pickup height is not where you want it, there’s no point setting intonation (and it’ll be more difficult). Get everything else right first and then look at intonation.
There are a few things going on here but for the most part, think of it this way:
When you fret a note on a guitar (or bass—let’s assume we’re talking about either instrument for the rest of this article), you actually stretch the string a little. This stretching sharpens the note by just a little.
To compensate for this—and you’ll often hear intonation referred to as ‘compensation’—we make each string a little longer, effectively flattening it.
Heavier strings will need more compensation than lighter strings. This is why you can see an acoustic guitar saddle angles back as it goes from treble to bass strings.
On most electric instruments, you’ll likely have adjustable saddles to easily compensate each string. Some guitars are more or less flexible in this regard (and we'll get to individual instruments in a little while) and acoustic instruments with non-adjustable saddles present more of a challenge.
Before we start, remember the prerequisites: The rest of your setup must be right for you and you should have fresh strings installed, properly stretched, and tuned up as normal.
Also, you'll always check with the instrument in the 'playing position', not resting on its back on a bench or similar. The instrument should be orientated as if you were playing it. Ideally, sit comfortably and hold the instrument in your lap like you would if you were playing. Feel free to set it down on a bench or whatever to actually make the adjustments but always back to playing position to check.
The basic theory is this:
If I affect my best Morgan Freeman voice, I can say something like, "Time was, back in my day, we set intonation using our ears."
Since this isn't back in the day, though, I'm going to assume you'll probably be using an electronic tuner of some sort. This can certainly help matters but try to make sure you use a decent one.
When you’ve got Peterson selling a strobe tuning app, with 0.1 cent accuracy for iOS and Android that costs about USD10, there’s no excuse. There are other apps and, of course, good hardware tuners too. Just try to use something decent.
Clip-on tuners aren't great for this job. Their accuracy is often lower and, more importantly, they sometimes have problems sensing the fretted notes. If it's all you've got, give it a go, but see if you can scrounge up something non-clippy if possible.
Or use your ears.
I've written a new book to accompany this series of posts on intonation. Solving Intonation Problems will give you some help dealing with the trickier issues you might come across. There are tons of tips and some info on some hardware solutions for improved intonation.
Solving Intonation Problems is a free download. Just sign up below for your free copy.
If you've a guitar or bass with through-body stringing, take care if you're doing any work on it with the strings removed. It's not that uncommon for one or more of the string ferrules (those little metal cups where the string anchors) to be loose. After crawling around, searching my workshop floor for the hundreth time, I began to stick a strip of masking tape over the ferrules as soon as the strings are removed.
Save yourself some hands-and-knees searching. Tape 'em up.
If you've got an older instrument be careful of the finish. Stick the tape on your jeans a few times to remove some of the tackiness and don't leave it on the guitar too long.
This can affect any instrument with string ferrules but Telecasters do seem most prone to disappearing ferrule syndrome.