  The K-matrix method is often used to describe overlapping resonances. It
guarantees the unitarity of the scattering matrix but its parameters are not
resonances masses and widths. It is also unclear how to separate resonant and
background contributions and to describe background in terms of phase shifts.
The Breit-Wigner (BW) approach operates with parameters having direct
experimental meaning but a simple sum of the BW functions is not unitary. We
show how to construct the unitary S-matrix by taking into account the
interference of the BW functions. The method is simple and straightforward,
background can be added to resonance amplitudes in the standard quantum
mechanics form. In examples we give a comparison between the K-matrix and
unitary BW-approaches.
