  It is argued that many nonextremal black branes exhibit a classical
Gregory-Laflamme (GL) instability. Why does the universal instability exist? To
find an answer to this question and explore other possible instabilities, we
study stability of black strings for all possible types of gravitational
perturbation. The perturbations are classified into tensor-, vector-, and
scalar-types, according to their behavior on the spherical section of the
background metric. The vector and scalar perturbations have exceptional
multipole moments, and we have paid particular attention to them. It is shown
that for each type of perturbations there is no normalizable negative
(unstable) modes, apart from the exceptional mode known as s-wave perturbation
which is exactly the GL mode. We discuss the origin of instability and comment
on the implication for the correlated-stability conjecture.
