“Movable” in this case just means “having no open strings.” If you know a movable major chord, you could use that one form to play G major, Ab major, C# major, etc. Learning one movable chord form–and how to use it–is better than memorizing ten open chords.
To get even more mileage out of movable forms, you can learn how to move them across strings as well as up and down one string. This takes just a little extra effort to pull off, and the small learning curve is completely worth the effort.
The Hump: Your Ornery ‘B’ String
Standard guitar tuning (EADGBE) is just a little bit musically askew. Most of the string pairs are the same musical distance from each other: the musical difference in pitch between E and A is called a fourth, for example. The difference in pitch from A to D is also a fourth. Same with D and G. But the distance from G to B is actually just a little bit smaller than any of the other string pairs listed above. If we want to move chord forms across the strings, we’re going to have to adjust slightly for this difference, just as we do when tuning a guitar by ear.
Fortunately there’s a simple way to visualize this. For purposes of moving chord forms across the strings, I think of the little difference between the G and B strings as “The hump.”
How to Move Chord Forms Across the Strings (and Across the Hump)
Let’s take a look at a simple movable chord form, one of the first jazz chords typically taught to guitarists who’re studying the genre.
The root of this chord is currently on the lowest (thickest) string. Let’s move the root—and all of the other notes–one string toward the floor. Because of the “hump,” we’ll have to make a small adjustment to the note that’s moving from the G string to the B string—by also moving it up one fret.
So as you can see, all you have to do is move all your fingers one string toward the floor—and the finger that moves from the G string to the B string also must move up one fret. That’s all there is to it!
Of course you may have a chord that you want to move one string toward the ceiling. In that case, just reverse this process: the finger moving from the B string to the G string should also move one fret toward the headstock. Easy, right?
The next step of course is to go try moving some chord forms across the strings as described above. In case you thought all the different ways to play major chords were entirely arbitrary, check out what happens when I move this barre chord across the strings: