lowered by two semitones
In Open-G tuning, simply use one finger to hold down all the strings on
any fret you wish, and you will get a major chord. This makes songs that
use lots of major chords very easy to play. It also allows the lower strings
to be used to play very "droning" bass notes. Both these features are useful
in 12-bar blues, so many blues artists have used Open-G tuning over the years.
String 6: E (lowest)
Open-E tuning is different from Open-G tuning in a few ways:
First, in Open-G tuning, blues songs in the key of G (and, to a lesser degree, the key of D)
are very easy to play. But in Open-E, the key of E is the easiest key to play.
Secondly, in Open-E, at any fret, the highest string yields the root note of
whatever chord that fret plays. This causes the chord to have a different "feeling" than
in Open-G, where the chord is inverted differently. The difference is hard to put into words;
to me, Open-E "feels" brighter and more upbeat, while Open-G "feels" a little more "brooding".
The two tunings create distinctly different moods and feelings in a song, and it is up to
you to decide which you like better for any given song. Personally, I like Open-E better
for more "retro"-style Delta blues, and I play it on a Telecaster to get that gritty twangy
tone that fits Delta blues nicely. For other forms of blues, like Chicago blues, I like
Open-G. But this is just my own personal preference. It is highly subjective.
Back to questions
The use of slide allows the guitarist to be very expressive, mimicking the
human voice or invoking deep emotion or strong "bluesy" energy. Slide guitar
can be done with a very clean guitar sound, yielding a very mellow, bluesy
feeling, or it can be used with a more "rock-n-roll" distorted sound, which
adds an exhilirating edge to blues-rock songs.
Slide guitar is also referred to as "bottleneck slide" because the broken-off
neck of a bottle was often used as a slide in the early days of blues.
The "diddley-bow" used a slide out of neccessity, since it had no frets. But
the use of a slide was soon considered a "neccessity" for guitars too. In the
sticky, humid heat of the deep South, the weather wreaked havoc on the tuning
of guitars, making the frets almost useless. Slides began to be used with guitars
simply to be able to play a note that was in tune! But as playing styles evolved over
time, the really cool sound became another good reason to use a slide.
Some of the earliest slide recordings were made by
Robert Johnson in the late 1930s.
Decades later, Muddy Waters popularized
the use of slide again, making it his own personal trademark and inspiring an
entire generation of blues musicians after him (including me).