Intonating a flattop’s bridge saddle is tricky, and that’s especially true when it comes to vintage 12-string guitars. (electric guitars to be covered later) High Action: An acoustic guitar generally has a fixed saddle (of bone or whatever). Compare each open string the harmonic at the twelfth fret. Causes of Poor Intonation : Incorrect positioning of the bridge or saddle: This causes the strings to be either too long causing intonation to be flat or too short causing it to play sharp.On an acoustic guitar this can be caused by the saddles crown or the actual position of the saddles slot. Intonation is another extremely important factor. Constant fluctuations in intonation are exactly why most guitars are designed with adjustable intonation mechanisms. PG's Andy Ellis talks with guitar repairman and setup technician John LeVan of LeVan's Guitar Services in Nashville, TN, about how to properly set up and intonate a flattop acoustic guitar. In fact for many players, the slightly out-of-tune jangle was a big part of a 12-string’s aural mystique. An acoustic with poor intonation will always sound out of tune. You’ll only need a few basic tools to do the job: a screwdriver, flat- or cross-head, depending on your guitar; the necessary tools to set string height, such as an Allen key; and a sensitive guitar tuner, preferably a strobe tuner if you have one. Compensating to correct the intonation: Part I This page is dedicated to working with intonation on acoustic guitars, but the information here is applicable to other fretted string instruments. Back in the day, it was rare to find an acoustic 12-string with a compensated bridge saddle. While tuner apps aren’t highly accurate, there sufficient enough to conduct this test. The fact that the saddle is installed at a slight angle (increasing string length from 1st to 6th) string is a nod towards some string compensation. Note: adjusting the intonation of an acoustic guitar usually requires careful filing of the saddle and is best left to professional guitar techs. Acoustic guitars offer a bit more of a challenge than most electrics though. Acoustic guitars are different and don’t give you an easy way to adjust the intonation. Much of my own understanding of intonation has evolved from long … To test the intonation of a guitar, use a tuner-app. You can buy a ‘compensated’ acoustic saddle as shown below that offers some slight adjustment for intonation across each string. In the above photo, you can see that there is no way to adjust the position of the bridge saddle. Intonating a guitar puts it in tune with itself by slightly shortening or lengthening each string , which is done using the adjustable string-length mechanisms found at the bridge .