Canadian folk legend Joni Mitchell certainly loves altered tunings. She utilises so many, in fact, that she's devised a system to remember which is which.
Let's start with standard tuning, EADGBE - she would remember it as E55545. That means, start at low E then hold down the ﬁfth fret to obtain an A note with which to tune the fifth string at. Then depress the ﬁfth string at the ﬁfth fret to produce a D note - the fourth string would then be tuned to this. An advantage to this method is that it's easier to see the relationships between tunings. For example, it's not obvious that DGCFAD is just standard dropped down a whole tone, but written as D55545, everything becomes clear.
How to get the tuning is one thing, what to actually play with it is another. Certainly, her use of altered tunings enables her to create new chord voicings and drones with fresh perspectives outside of standard tuning. Try out this acoustic piece, modelled on some of the songs found on her classic Blue album. It uses a gorgeous open G tuning (DGDGBD), or rather D57543 in 'Joni' tuning.
Joni Mitchell has used Martin D-28s for most of her career. We used a smaller Tacoma PM-28 for the recording, mic'd with an AKG C1000S into a laptop via an Edirol UA-25 external soundcard.
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Audio - Tutorial track