What tuning does a7x use on nightmare?

Answer 1

Avenged Sevenfold loves to use Dropped D tuning in general.

In the song "Nightmare," the Guitarists use 'Dropped D' tuning.

From the 'thickest' string to the 'thinnest' string: D.A.D.G.B.E

This song is especially characterized by the rhythmic riffs that accentuate the shifting power chords. The 'Dropped D' tuning provides a convenient way to change between such power chords efficiently and accurately.

Similarly, the bassist uses a 'Dropped D' tuning as well.

From the 'thickest string to the 'thinnest' string: D.A.D.G.

As an extra note: Make sure you tune the guitar/bass well; it is always good to have instruments in tune, especially in this song with the changing power chords!