  The correspondence between BRST-BFV, Dirac and projection operator approaches
to quantize constrained systems is analyzed. It is shown that the component of
the BFV wave function with maximal number of ghosts and antighosts in the
Schrodinger representation may be viewed as a wave function in the projection
operator approach. It is shown by using the relationship between different
quantization techniques that the Marnelius inner product for BRST-BFV systems
should be in general modified in order to take into account the topology of the
group; the Giulini-Marolf group averaging prescription for the inner product is
obtained from the BRST-BFV method. The relationship between observables in
different approaches is also found.
