D G C F A D -- 'Dropped Standard' / 'whole-step down' Paul McCartney,
for Yesterday, Neil Young, for Tell Me Why and Sugar Mountain 
[correct?!], R.E.M.'s Peter Buck, Elliott Smith, many others. 

C A D G B E -- Drop C 6th string down to C, and everything else as in 
Standard tuning. Bob Dylan, for It's All Over Now, Baby Blue. 
- Acoustic Guitar magazine, Dec 2000

C G C G C E -- Open C Bill Frisell, who wrote the tune "Brother" with 
experimenting with this tuning. "...the guitar would ring with these 
wonderful overtones" - Acoustic Guitar magazine, Sept 1997

C G D G B E -- Drop-C Duck Baker, John Leventhal... "You keep the top 
four strings the same so you can sort of voice chords the way you're 
familiar with doing it, and then it gives you a lot of rich root-fifth
possibilities underneath it." - John Leventhal (Works OK with my 
light-gauge strings.) 

C G D G A D -- C6add9 This tuning can be thought of as DADGAD with the
5th and 6th strings dropped yet another whole step. Lawrence Juber 
(and others?).Juber sometimes switches to it from DADGAD to play a 
song in C. 

C G C F A D -- Drop-C This tuning is not as strange as it looks. 
It's just Drop-D, with all strings tuned one full step lower, so the
relationships between the strings (except for the 5th and 6th) are 
the same as in standard tuning. Joseph Spence, Neil Young, Leo Kottke 
- studio version of Louise. Kottke has also performed his Louise live,
tuned half a step lower yet - in Drop-Bb... not on light gauge string,
I expect!

C G C F C D -- Csus4add9 Invented by Martin Simpson, who uses it 
especially for slide playing. 

Eb Ab C# F# Bb Eb - one half step low Many players, including Eddie 
Van Halen, on Jamie's Crying'. 

Most of these 'D tunings' above can be arranged / thought of as a 
series of progressive variations departing from Standard tuning one 
step at a time. See the illustration below. Each new tuning drops one 
and only one of the strings, as indicated by the notes colored red. 
The strings are generally lowered a whole step, except in the case of 
the 3rd string, which drops by half steps to open up six string D 
major and D minor chords, in turn. 

6 5 4 3 2 1 
·	E A D G B E -- Standard
·	D A D G B E -- Drop D 
·	D A D G B D -- Double Drop D 
·	D A D G A D -- DADGAD 
·	D A D F# A D -- Open D 
·	D A D F A D -- Open Dm 
·	D A D E A D -- Dadd2 
·	Here is nearly the same sequence, but with Phil Keaggy's D6 
        tuning (below) taking the place of DADGAD in the fourth slot:


6 5 4 3 2 1 
·	E A D G B E -- Standard
·	D A D G B E -- Drop D 
·	D A D G B D -- Double Drop D 
·	D A D F# B D -- D6 
·	D A D F# A D -- Open D 
·	D A D F A D -- Open Dm 
·	D A D E A D -- Dadd2 
·	Note that the 5th and 4th strings remain unchanged throughout 
        both sequences, since they are already the 5th (A) and 1st 
        (D) degrees, respectively, of the key of D.