  A new tool for the investigation of 2+1 dimensional gravity is proposed. It
is shown that in a stationary 2+1 dimensional spacetime, the eigenvectors of
the covariant derivative of the timelike Killing vector form a rigid structure,
the {\it principal Killing triad}. Two of the triad vectors are null, and in
many respects they play the role similar to the principal null directions in
the algebraically special 4-D spacetimes. It is demonstrated that the principal
Killing triad can be efficiently used for classification and study of
stationary 2+1 spacetimes.
  One of the most interesting applications is a study of minimal surfaces in a
stationary spacetime. A {\it principal Killing surface} is defined as a surface
formed by Killing trajectories passing through a null ray, which is tangent to
one of the null vectors of the principal Killing triad. We prove that a
principal Killing surface is minimal if and only if the corresponding null
vector is geodesic. Furthermore, we prove that if the 2+1 dimensional spacetime
contains a static limit, then the only regular stationary timelike minimal
2-surfaces that cross the static limit, are the minimal principal Killing
surfaces.
  A timelike minimal surface is a solution to the Nambu-Goto equations of
motion and hence it describes a cosmic string configuration. A stationary
string interacting with a 2+1 dimensional rotating black hole is discussed in
detail.
