The first real open tuning you will learn is open G tuning. Open G tuning is the same as dropped G tuning except the first string (E) is also lowered one tone to the note D. The open string notes as shown in the following diagram will therefore be, beginning with the sixth string;
These notes form a G major chord. A G major chord consists of the notes G, B and D. For more information on chord construction see the Appendices.