For blues guitar players getting into the realm of alternate tunings, there is a lot of confusion about the different tunings. I see this all the time in online forums where the focus is open tuning or slide guitar. "Open E", "Open D", "Open G", and "Open A" are frequently used to describe tunings, however this can be very confusing for players just starting out. Most experienced players use terms that describe the intervals rather than the key. This is a simple concept that most guitar players should be able to understand. An interval, in the context of a guitar tuning, is typically describing a chord shape that is made when playing in standard position. For example, an E chord looks like this:
We refer to an open tuning that forms this chord as Vestapol tuning. When tuning a guitar to "Open E", you would raise the A and D strings a full step, and raise the G string a half step. This allows you to play an E chord by simple strumming the open strings.
The confusion comes when someone describes and "Open D" tuning. This is the usually the SAME tuning -- Vestapol. The difference is, instead of tuning 3 strings UP to reach the intervals that allow the open chord to ring out, you tune 4 strings DOWN -- Low E string is tuned down to D, A and D strings left as A and D, G string is tuned down to F#, and B and E strings are tuned down a full step to A and D. This allows you to play an open D chord (using the E chord shape) by simply strumming the open strings. The notes are D A D F# A D. There are other "open D" tunings, which is confusing, but if you think of the tuning as a chord shape (intervals), then it becomes more clear.
Open E tuning will cause additional stress on your neck and top (on an acoustic), which is why most people will tune to D and capo up a few frets to change keys. However, this presents a problem for slide players -- if playing in keys like F# or G, sliding up to the high octave requires that you slide up to frets 16 or 17 (12 frets up from your capo at 4 or 5). Tuning to Open E makes it easier to play keys like F# by capoing up only 2 frets, allowing you to slide up to fret 14 with relative ease. The notes are E B E G# B E.
The same principle applies to the A chord, which forms the intervals for Spanish tuning. Spanish tuning can refer to "Open A" or "Open G", or any other key that relies on the A chord shape.
For Open G tuning, you would tune the E, A, and high E strings down a full step. This gives you a G chord when you play the open strings, but is the same as playing an A chord tuned a full step down. The intervals are the A chord shape. The notes are D G D G B D.
For Open A tuning, you tune the D, G, and B strings UP a full step. This gives you an A chord when playing the open strings, and is the same as playing an A chord in standard tuning. The notes are E A E A C# E. It should also be noted that tuning the strings higher (to an A chord) can cause additional stress on your neck and top (on an acoustic).
Open A tuning will cause additional stress on your neck and top (on an acoustic), which is why most people will tune to G and capo up a few frets to change keys. However, this presents a problem for slide players -- if playing in keys like B or C, sliding up to the high octave requires that you slide up to frets 16 or 17 (12 frets up from your capo at 4 or 5). Tuning to Open A makes it easier to play keys like B by capoing up only 2 frets, allowing you to slide up to fret 14 with relative ease.
Guys like Robert Johnson most likely tuned up to Open E and open A (Vestapol and Spanish, respectively) because it would have been harder to tune down and play up at the 16th fret. One of the only known pictures of Robert Johnson shows him with a capo at the 2nd fret.
An E-minor chord shape produces a tuning known in some circles as "Cross Note" tuning, or "Bentonia" tuning (after the area where Skip James lived). This tuning can refer to D-minor, E-minor, and other similar minor keys, all using the E-minor chord shape.
To produce the open D-minor tuning, you would tune the low E, G, B, and high E strings down a full step to D, F, A, and D. This is the tuning that Skip James used in his historic recordings of 1931. The notes are D A D F A D. Bukka White also used a Cross Note tuning somewhere between C# and D minor, however he played mostly in a major key by fretting the 3rd string at the first fret.
To produce an E-minor tuning, you would tune the A and D strings up a whole step to B and E. This gives you the same E-minor chord that you would get by fingering the chord in standard position (as shown in the diagram).
It should be noted that any of these tunings work in other keys by tuning to the open chord. For example, a Spanish tuning in F# can be made by tuning the guitar down to the open F# chord (C# F# C# F# A C#). A Vestapol tuning in C# can be made by tuning the strings down to C# G# C# F G# C#.
The Tunings:
D A D F A D -- Cross Note (D minor)
E B E G B E -- Cross Note (E minor)
D A D F# A D -- Vestapol (open D)
E B E G# B E -- Vestapol (open E)
D G D G B D -- Spanish tuning (open G)
E A E A C# E -- Spanish tuning (open A)
There are other tunings as well (such as drop D, which does not form a chord), but these are the main tunings used by the old blues guys and slide players.
Update (9/7/22): You can get all keys using these tunings by using capo and/or adjusting the pitch of the strings. Capo at the second fret is excellent for slide guitar playing on a 14 fret guitar (14th fret intersecting the body) whereas playing in open position is great on a 12 fret guitar (12th fret intersecting the body). The following chart shows tunings and capo positions, with a couple of alternate variations.