Many vibrato systems can be set up in such a way that they allow for changing string pitch both up and down. Eddie Van Halen prefers instead to have his set up so it is flush with the guitar body, which has two advantages: first, a broken string doesn't effect the pitch of the other strings, and it can accommodate attachment of a device called a D-Tuna to the bridge. This device can drop the low E-string down a whole step to D to extend the tonal variety of the guitar, even during live performance.