I’m going to play a D major scale in the most boring way imaginable. Any time I'm teaching, I ask students to say what they hear differently and immediately I get either harp or church bells—both of which are correct. My friend Pierre Bensusan, who is associated with this tuning more than any guitar player alive, coined the term “harpisant” or “harplike” and though in classical guitar I'm not aware of any use of DADGAD tuning, there is a term for the concept of playing across the strings—“campanella” which means “bell like." The idea is to let notes RING!
So let’s look at that scale in detail starting on the open top string. Then, we have 4th fret on the second string and then 4th fret on the third string. Now, the first thing to mess up your head—we're going to go UP a string, from the 3rd to 2nd string to go DOWN a note.
Open strings in any context give the guitar player a chance to change position. We have two open strings in a row, which allows us to get from the starting position to the end position. The point of the weird looking fingering is to create the scale across the strings. We have four notes of a scale across four adjacent strings and on two occasions we go up a string to go down a note. Get used to it—it’s a really handy tool to have at your disposal; to have an open string to get us down the neck.
It’s just a major scale, nothing fancy, but there’s something really satisfying about playing that scale. You can feel the guitar vibrate! Look at the strings you use in playing it the boring way: 2, 2, 2, 2, 3, 4, 4, 4. Now here’s the cool way—it goes: 1, 2, 3, 2, 3, 4, 5, 4. You don't use the same string twice in a row. If you get fancy and extend it you can go 1, 2, 3, 1, 2, 3, 2, 3, 4, 5, 4, 5, 6, 5, 6.
So, the point of this exercise is not to be able to play scales, but to have fragments of that string of notes at your disposal. Those fragments are like words—you're increasing your vocabulary and when arranging melodies bits of that scale will crop up relentlessly, so it’s a good idea to have it at your fingertips in a way that you can play it automatically.
Then if you're feeling really adventurous try applying that idea the key of Bb. The open strings have different roles in this key, but we're going to stick mainly to the key of D. Have that scale under your fingers because it’s going to be really useful!