The quarter-tone slide is a little tricky to describe in print, and you may find it difficult at first to hear the difference between a quarter-tone slide and a half-tone slide. To begin with, remember that a one-fret distance on the guitar is called a half step or a half tone. So a quarter tone is just going to be half of that. You might be wondering, “How am I supposed to play half a fret?” That’s where the slide comes in. On the third string, slide from the third fret completely up to the fourth fret (Example 1a). That’s a half-step (one-fret) slide, and if you play that over a G bass, the B you’re landing on will sound really bright and perfect as the major third of a G chord.
Now slide from the third fret only halfway up to the fourth fret—to an imaginary three-and-a-half fret (Example 1b). Over a G bass, that note won’t quite sound major, and it won’t quite sound minor. If you just stop right there in the middle, it won’t quite sound right, either. There’s a blue note you’re trying to find, and I can really only describe it as being the note on the way to the major third. If you haven’t lost your patience with this project yet, try this last step: Start at the third fret again and slide toward the fourth fret, but this time lift the slide from the strings somewhere between your start at the third fret and your conclusion at the fourth, killing the note midflight. You need to be damping behind the slide with your index finger so that lifting the slide actually stops the note; otherwise you’ll get a pull-off to the open G.
Excerpted from Acoustic Guitar Slide Basics