Open A major — a bright, high-energy slide and blues tuning. Strum open and you get an A chord. Low to high: E A E A C# E.
Open A tuning is, from the low (6th) string to the high (1st) string, E A E A C# E.
Start in standard tuning (E A D G B E). Raise the 4th string (D) up a whole step to E. Raise the 3rd string (G) up a whole step to A. Raise the 2nd string (B) up a whole step to C#. Result: E A E A C# E — strum open for an A major chord. Note this raises three strings, so tension goes up; many players use lighter strings or tune the low strings down to a Drop-style open A instead.
Open A major — a bright, high-energy slide and blues tuning. Strum open and you get an A chord.
More tunings: see all 15 tunings · standard tuning chart