A general rule for guitarists who want to change from standard to an opening tuning is to tune down for acoustic guitar and up for electric. The two most popular open tunings, especially for slide, are Spanish and Vestapol, named after tunes once popularly played in those tunings. Spanish tuning is Open G or A. Vestapol is open D or E. You can tune lower or high and still be in one of these two tunings. Following the example of Leo Kottke and others, I often tune my 12-string guitar down so the first and sixth strings are both C# and sometimes to C (though that’s a bit loose and floppy). Spanish tuning thus becomes open F (or E if 1 & 6 are C) and Vestapol is Open C# (or C).
The logic of tuning up on an electric is because most modern electric guitarists use light gauge strings. The tension on the strings if tuned down to D to D would not be high enough to prevent the strings from buzzing. It would be difficult to keep a slide from pushing the strings down to the frets.
The consideration for an acoustic guitar is too much tension if tuned up to open A or E. If your acoustic isn’t built to support it, the extra force could harm your guitar. Equally important is that the action on an acoustic guitar is a bit higher than on an electric. The extra tension from tuning up will make it more difficult to play fretted notes.
If you own a Fender and a Gibson (e.g. a Stratocaster and a Les Paul) and string both with the same set of strings, and tune both guitars the same (regardless of tuning), the tension on the strings for the Gibson will be 94.2% of the tension on the Fender (i.e., nearly 6% less).
Tension Changes with Pitch
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Tuning down 2 half-steps will always result in a tension that is 79% of the original, regardless of the starting note. If you know the starting tension from the manufacturer’s product information, you can calculate the tension for alternate tunings using the above table.
What if you want to put the strings you use on your favorite electric guitar on a lap steel guitar with a 22.5 inch scale length? If you’re comparing to a Strat, the tension on the lap steel guitar would be about 78% of the Fender (~22% lower than the Strat). Now combine that with tuning down to open G or open D. If you’ve been using a 9-42 set, you’ll have some seriously floppy strings on the lap steel.
L: Scale length in inches. % T: Percent change in tension for identical strings and pitch at different scale length as compared to 25.5 inches on Stratocaster.
You can find string and set tensions on the D’Addario website. Not all manufacturers provide string tension information. Stringjoy has a nice tension calculator that allows you to make and order custom sets of strings. The tool includes the ability to set scale length as well as explore the effect of different string gauges on tension.