Drop D tuning
The Drop D tuning is perhaps the most common altered tuning and it's a handy one since only one string is changed.
It can be used in many styles and is especially popular in hard rock and one reason for that is that it allows power chords to be played by placing a single finger over the same fret. Drop D tuning should not be confused with Open D tuning.
Here is how you tune the guitar: DADGBE. Compared to standard tuning, the only difference is that you tune down the lowest (thickest) string a whole step, from E to D. Now you have a guitar that delivers more bass in the sound; perfect for some rock styles as already mention, but there are other possibilities as well. There is also the Double Drop D.
There are two alternatives for playing power chords in this tuning. Here are examples of chords with both shapes, which obviously are movable.
The popularity of this tuning depends much on the many heavy and cool riffs that can be easily created by using power chords in alternation with the open D5. A video that demonstrates this. Here's a chord progressions, meant to play raw and in a fast tempo:
D5 - D5(XII) - D5 - C5 - D5(XII) - D5 - C5 - C#5 - D5(XII)
Chords with alternative bass notes
Power chords with alternative bass notes and suspended chords alike are very useful and can be used together with the regular power chords presented above.
The song "Everlong" by Foo Fighters uses three of the above chords in the verses:
F#5/D - F#sus/B - Dsus/G - F#sus/B
Since only the lowest string differs from standard tuning, many open chords could be played just as normal. Although, rich sounds can be found by include the open strings, especially for D chords. For chords that otherwise use the sixth string, such as E and G, there are some new conditions (E major is by the way not very common in songs using this tuning).
Em can also be played by using three fingers or the thumb. An alternative way to play Em is xx1000. The G chord can also be played as 020003.
Additional open chords in Drop D:
E: 222100 / XX2100
G: 5X0003 / 550003
Using shapes around the middle open strings
The 3rd, 4th and 5th strings played open forms a D chord. You can create shapes around these three open strings, some examples are:
In Drop D tuning, some new opportunities for blues guitar turns up. These are some chord ideas for playing blues in the key of D.
The shape for G9/D will result in G7/D played as XX0001.
The conditions for barre chords changes when the lowest string is included.
Common barre chords that are played differently:
Progressions in this specific tuning:
D - C/D - A/D
Dmadd2 - Fmaj13/D - C/D
Aadd9 (777600) - D6add9 (12 12 12 11 0 0) - G6 (555400) - Aadd9/Gb (444200) - Esus4/D (000200)
Although Drop D are especially suited for playing in D major key it should not stopping you from playing in other keys as well, for which new ideas can emerge. It expands the bass register and can sound very cool.
If you want to go more into depth of this particular guitar tuning, see the Essential Chords in Drop D Tuning ebook with over 100 chord diagrams.
This was an introduction about chords and playing ideas in the Drop D tuning, see also Drop C tuning.