  The standard method used for tagging b-hadrons in the DELPHI experiment at
the CERN LEP Collider is discussed in detail. The main ingredient of b-tagging
is the impact parameters of tracks, which relies mostly on the vertex detector.
Additional information, such as the mass of particles associated to a secondary
vertex, significantly improves the selection efficiency and the background
suppression. The paper describes various discriminating variables used for the
tagging and the procedure of their combination. In addition, applications of
b-tagging to some physics analyses, which depend crucially on the performance
and reliability of b-tagging, are described briefly.
