Drop D tuning:
Drop D tuning, also known as D-A-D-G-B-E is an alternate, or scordatura, form of guitar tuning.
Specifically - a dropped tuning - in which the lowest (sixth) string is tuned down ("dropped") from the usual E of standard tuning by one whole step / a tone (2 frets) to D.
An open-G tuning D-G-D-G-B-D allows a G-major chord to be strummed on all six strings with neither fretting of the left hand nor a capo. Like other open tunings, it allows the eleven major chords besides G major each to be strummed by barring at most one finger on exactly one fret.
The discovery of open G tuning was a revelation for Keith Richards, who wrote about the experience with awe and reverence in his autobiography Life.
Open G helped Richards create a personal sound and approach on guitar, and yielded such classic Rolling Stones songs as “Honkytonk Women,” “Brown Sugar,” “Beast of Burden,” and “Start Me Up.”
E♭ (E‐flat) tuning or D♯ (D‐sharp) tuning is an alternative form of guitar tuning. It is based on the standard guitar tuning, except that all of the strings are lowered by one half‐step. The result looks like this: E♭‐A♭‐D♭‐G♭‐B♭‐E♭. The guitar is usually played identically to standard tuning, resulting in a heavier tone without requiring the guitarist to learn different chords or fingering.
The tuning has a number of other features attractive to artists. It allows the use of heavier strings without decreasing playability. The tuning is also easier to play with a saxophone accompaniment (saxophones are usually tuned to B♭ or E♭). It can also be matched with the E♭ pentatonic minor scale formed by the black keys of a keyboard.
Blues legend Robert Johnson was one of the first to experiment with alternate guitar tunings such as E♭. Jimi Hendrix was also famous for tuning down a half-step, and Stevie Ray Vaughan used it almost exclusively in his electric guitar playing, as did Slash of Guns N' Roses fame. The tuning remains very popular in rock and blues music, although some (like Pantera) tune down a quarter of a step.