There are two basic tunings...the "E"...
(highest pitch to lowest)...A "D" tuning can be used by just transposing down a whole step...
1st: E
2nd: B
3rd: G#
4rh: E
5th: B
6th: E
The "double G" or "double A"(just transpose up a step)....
1st: D
2nd: B
3rd: G
4th: D
5th: B
6th: G
For example, "Steel Guitar Rag" is based on the "E" tuning....the "double G" is the common Dobro tuning for bluegrass and old time country. The "double A" could be used for bluegrass and blues.