The open strings of the six string guitar in the standard tuning are tuned to E, A, D, G, B, and E lowest to highest. Many alternative tunings are available. Some examples are provided below.
Double drop D tuning – D, A, D, G, B, D
Drop D tuning – D, A, D, G, B, E
Modal D tuning – D, A, D, G, A, D
Modal G tuning – D, G, D, G, C, D
Open D tuning – D, A, D, F#, A, D
Open A tuning – E, A, C#, E, A, E
Open G tuning – D, G, D, G, B, D
All fourths tuning – E, A, D, G, C, F
The examples above can be extended. In each of the examples above, tuning each string up and down half-a-step or more, or using a capo, would produce other tunings. Alternatively, taking the open major chord tunings and dropping the third of the relevant chord half-a-step down would produce open minor chord tunings (e.g., open D minor tuning – D, A, D, F, A, D and open G minor tuning – D, G, D, G, Bb, D). Taking the minor or major third and dropping it to a major second or raising it to a perfect fourth would produce modal tunings (e.g., raising the second string B in the open G tuning to a C produces the modal G tuning). Other tunings unrelated to the examples above can also be used (e.g., all fourths tuning – E, A, D, G, C, F, where A is the perfect fourth of E, D is the perfect fourth of A, and so on, allowing the easy moving of chords from low to high or from high to low strings).