From Jimmy Page to Joni Mitchell and Keith Richards, as well as most bluesmen and metalheads, many guitarists resort to these alternative tunings to get new harmonies out from their guitars or to simply make their lives easier. Here is a brief overview of the most celebrated ones, which you should try out if you have already explored all possibilities of standard tuning.
D A D G B E - Drop D
Extremely often used by metal axemen, it consists in tuning down the bottom E string to get a D. The dropped down string renders a deeper sound and together with the A and D open strings it forms a very heavy power chord, very practical when it comes to aggressive riffs. It is worth noting that several metal players haven't shied from tuning down to sharp C in order to get a heavier and deeper sound, and even to C and B, giving birth to Drop C#, C and B tunings.
Songs using Drop D: Black Hole Sun (Soundgarden), Grace (Jeff Buckley), Moby Dick (Led Zeppelin), Dear Prudence (The Beatles), Everlong (Foo Fighters), Killing In The Name (Rage Against The Machine), and many songs by Linkin Park, Deftones, System of a Down, Pantera…
D A D G B D - Double Drop D
In the wake of Drop D, many guitarists also lowered the high E string to a D (DADGBD). This type of tuning is called "Double Drop D".
Songs using Double Drop D: The Loner (Neil Young), The End (The Doors), Going to California (Led Zeppelin), It’s alright, Ma (I'm only bleeding) (Bob Dylan)…
D A D F # A D - Open D
Tuning down the two E strings and the B string one whole tone, and lowering the G string a semitone you get a very nice D major chord when playing open strings. A tuning that has brought happiness to many a bluesman. It is indeed very popular with bottleneck players. This tuning can also be converted into a D minor chord by tuning down the G to F (DADFAD), thus giving birth to… Open D minor (Open Dm).
Songs using Open D major: many songs by Elmore James and Robert Johnson, Chelsea Morning (Joni Mitchell), Scarborough Fair (Simon & Garfunkel), many songs on Bob Dylan's Blood on the Tracks, She Talks To Angels (The Black Crowes), Paris Texas (Ry Coorder)…
D G D G B D - Open G
In order to get a G major chord with open strings you should tune down a whole step the two E strings and the A string. It can also become a minor chord by lowering the B string a semitone (which would turn into an A# to end up with DGDGA#D). It is without a doubt one of the most utilized tunings by bluesmen and their direct heirs in rock music.
Songs using Open G: Honky Tonk Woman, Brown Sugar and even Start me up (Rolling Stones), That's The Way, Braun-y-aur (Led Zeppelin) and many blues songs by Son House, Muddy Waters, Robert Johnson, John Lee Hooker and even Rory Gallagher.
D A D G A D – D modal / Celtic
Widely used in Celtic music and called "my CIA tuning" by Jimmy Page, this tuning is at the core of many folk and rock songs thanks to its sounds evoking Scottich bagpipes and the drones that we often hear in North African, Indian and Arabic music. It can be obtained by taking a Double Drop D and tuning down the B string to make it an A.
Songs using Dadgad: Kashmir (Led Zeppelin), Circle (Slipknot), She Moves Through the Fair (Davy Graham), and lots of songs by Ani Di Franco
E A C # E A E – Open A
Producing a nice A major chord when strumming the open strings, Open A is achieved by lowering the D to C#, G to E and B to A. There is also a variant doubling on the C to make it EAC#EAC#. A tuning that can't be separated from the Delta blues and which lends itself pretty good to play slide guitar.
Songs using Open A: lots of songs by John Lee Hooker and Lightnin’ Hopkins.
E B E G# B E – Open E
Starting from the standard tuning, we get this Open E by tuning the A string up to B, the G up to G# and the D up to E.
Songs using Open E: Sitting On The Dock Of The Bay (Otis Redding), Statesboro Blues (Blind Willy McTell), Little Martha (Allman Brothers Band)
C G C G C E - Open C
The Open C tuning, which has a lot of variants, is achieved by lowering the bottom E string to C, the A to G and the D and B to C. It goes without saying that unless you want your lowest string to become a guitar strap, you'd better choose well the string gauge.
Songs using Open C: Breakin' Down (Ben Harper), Most High (Page & Plant), Friends (Led Zeppelin)
It would be impossible to list here all alternative tunings used by guitarists throughout the centuries. Take Nick Drake or Joni Mitchell, for instance, and you can find hundreds of tunings, including some extremely peculiar ones.
The point is that you shouldn't hesitate trying out your own tunings to get new colors, new harmonies, and especially to get rid of your habits and routines as a guitarist. By dint of playing, we end up using the same chords and the same progressions over and over again — and alternative tunings are surely the most simple way to put a remedy to that.