One of my favorite alternate tunings is Open C, or CGCGCE. As you can see, it’s mostly made up of a bunch of Cs and Gs in different octaves. What I really like about it is that it can be a brutal low tuning for aggressive chugging, but it also gives you a nice sense of atmosphere and additional harmonic complexity on the middle strings for prog metal. And it’s laid out on the fretboard in a way that invites some pretty interesting sweep picking patterns too. It’s pretty much the ultimate metal open tuning, at least for the stuff I play. Let’s look at some ways of spicing up your metal rhythm guitar with this tuning.
To really get to the heart of this tuning, let’s break it up into different zones. The first and most obvious is the bottom pair of strings. Tuned to C and its perfect fifth G, they give you a power chord (root + 5th) when you play both simultaneously. Let’s look at some musical examples (recorded with the Gus G Fire Blackouts set in an Ibanez RG).
Figure 1

Figure 1 is a riff which uses open and fretted power chords as well as exploring the relationship between octaves between these two strings: the octave of the open C note on the sixth string is found on the fifth fret of the fifth string, while the octave of the D# on the third fret of the sixth string is found at the eighth fret of the fifth string.
Figure 2

These intervals are repeated on the middle pair of strings, since they’re both also tuned to C and G: it’s just that now they’re an octave higher. This means we can play octaves by fretting two C strings or two G strings in the same location. Although you can use one finger to do this, I like to fret this type of octave with two fingers, which makes it easier to mute the string in between. Figure 2 is based on Figure 1 but with this type of octave used on the higher stabs in the second half of each bar.
Figure 3

The next set of intervals we have to work with are the perfect fourth between the fifth and fourth strings (again replicated an octave higher between the third and second strings). Since this is the same set of intervals used for most of the strings in standard guitar tuning, you can transfer your existing frame of reference right over to these string pairs. Figure 3 features some pretty standard pull-offs in the first two bars, while the third and fourth introduce an open C power chord on the bottom two strings, just to tie things in nicely and show you how the G+C string pairs compare to the C+G ones.
Figure 4

One of the coolest things about Open C tuning is set of stacked fifths available to you if you play a three-note power chord shape starting on the fifth string. If you were in standard tuning this chord shape would give you a root, a fifth and an octave. But in Open C you get an Sus2 chord, which can sound rather epic in conjunction with high gain and some delay.
There’s lots more you can do with this scale in a rhythm context, especially once you start taking two-string patterns and stretching them out across multiple octaves. And it’s easier to improvise melodies and accents using natural harmonics in this scale.