Guitar strings are typically tuned E-A-D-G-B-E from the lowest-pitched string to the highest. This is referred to as standard tuning. Some guitar players use alternate tunings, sometimes referred to as dropped tunings or open chord tunings, depending upon the type of music they play. One of the most frequently used alternate tunings, especially for slide players, is the key of D tuning, known also as the open D tuning.
Tune your lowest string, which is an E, first. Use a digital tuner to help ensure your strings are brought to the correct pitches. Turn your E tuning key counterclockwise to lower it one whole step. The string should read D on your tuner.
Leave the fifth string and fourth strings of your guitar as is. The fifth string is an A, and the fourth string is a D. These notes belong in the D chord and are part of your key of D tuning.
Lower your third string a half step, from G to F sharp. Do this by turning your tuning key counterclockwise to loosen the string. Use a digital tuner for accuracy.
Lower your second and first strings by turning the tuning keys counterclockwise to loosen them. The second string will go from B to A and the first string will go from E to D, leaving you with an open D tuning. This means that you will get a D chord by strumming all of your strings without fretting.
Alternately, some guitar players simply lower the lowest-pitched string from E to D, thus creating a dropped D tuning.