Joni Mitchell once said that she used abut 50 different guitar tunings in her performances, which is quite a lot for any guitarist. I wouldn't like to tune up and down several times in any one performance. Of course, it's best to have several guitar on hand for different tuning and styles, but not 50! It's probable that they were subtle variations on the basic open tuning patterns open D, open G and open C, which originally came from the Delta blues men in the deep south of the United States. The first guitars were not that great quality, even you got hold of a factory made one (guitars were often home made).
The Sears company revolutionized shopping with their mail catalog which happened to include basic Stella guitar models. They cost just one dollar, but even this was a stretch for many in the black community after the ending of slavery. The weather in the Mississippi Delta was hot and humid, a disastrous combination for an instrument made of wood, which tended to expand and contract making it very difficult to keep in tune. This is one reason why open G was popular. If one strummed the fingers across the open strings, then a G chord sounded. Similarly, if frets 5 and 7 are barred, a different chords is formed, and using these three, a basic blues could be created.
A favorite technique was to use a bottleneck or metal slide and slide it up to the required note with a little vibrato created by wobbling the finger holding the tube. This created a nice sound and also gave a little leeway to search for, and find, the right note without it sounding amateurish. This is how the typical delta blues sound was created. Other kinds of songs can be created by finger picking open G, but other chord shapes need to be learned as the ones used for normal tuning just won't do - see it here.
Three of the main exponents of this style (although there were many) were Robert Johnson, Son House and Muddy Waters. Muddy Waters later was a major force in Chicago style electric blues, but in his earl Mississippi days he was fantastic at playing bottleneck, claiming to have been playing 'Walking Blues', made famous by Robert Johnson, before Johnson himself recorded it. The older Son House was an obvious inspiration for both men and his powerful singing style coupled with the ringing bottleneck sound if his National Steel guitar was an incredible experience.
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