Open D tuning is an alternate tuning for the acoustic or electric guitar. The open string notes in this tuning are: DADF#AD. It uses the three notes that form the triad of a D major chord: D, the root note; A, the perfect fifth; and F#, the major third.
When the guitar is strummed without fretting any of the strings a D major chord is sounded. This means that any major chord can be easily created using one finger, fretting all the strings at once (also known as barring); for example, fretting all the strings at the second fret will produce an E major, at the third fret an F major, and so on up the neck.
Open D tuning is very popular with slide guitar (or 'bottleneck') players, as it allows them to play complete chords using the slide. This tuning is also used for regular (non-slide) guitar playing. The full and vibrant sound it produces - particularly on an acoustic guitar - also makes it ideal for fingerstyle playing.
Many new chord shapes and sounds are available with open D tuning. It can offer a strong compositional element that produce markedly different tonal qualities than standard tuning. The full range of major and minor chords, with all their extensions, are available to the player. Many well-known guitarists have used this tuning at some point in their career.
Open Dmaj7 tuning
This is lowering the high D string a half step to C# when tuned to open D. It is the major seventh of the D major scale, hence the name. It also can be used for both fingerstyle and slide guitar work.
(Courtesy of Wikipedia)
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