Open E Tuning

Struck open, the six strings ring as an E major chord, so a straight bar at any position plays a major chord. Five strings move from C6 to get there.

A major chord on the open strings

Open E tunes the six strings to an E major chord: E B E G# B E from the low string up. Struck open it rings as E major, and because a straight bar moves every string together, the bar plays a major chord at every position: A at fret 5, B at fret 7, E again at fret 12.

That simplicity is the point and the limit. A bar or a slant spells five chord types here, the major and minor triads, the power chord, the dominant seventh and the 6sus2, against C6's eight. The sixth and minor seventh sounds C6 is built around are not under the bar.

The retune

Five strings move from C6, and only string 1, the high E, stays. String 6 moves four semitones up, from C to E, and string 5 five semitones down, from E to B. Luna Guitars describes the tuning as open D a whole step higher: D A D F# A D raised two semitones on every string.

What moves from C6

Open E against C6, string by string, string 1 (the highest) first. Each move is the nearest one that reaches the new note.
String C6 Open E Move
1 E E stays
2 C B down 1 semitone
3 A G# down 1 semitone
4 G E down 3 semitones
5 E B down 5 semitones
6 C E up 4 semitones

Five of the six strings move. Struck open, the strings ring as E Major.

What a bar spells

Every chord type a straight bar or a slant spells in Open E or in C6. A type is spelled on all twelve roots or on none, because the bar slides.
Chord type C6 Open E
Major yes yes
Minor yes yes
Power (5) yes yes
7 — yes
m7 yes —
maj7sus2 yes —
6sus2 yes yes
6 yes —
m(add9) yes —

Open E spells five chord types to C6's eight: one gained, four given up.

Where this row comes from