  The importance of implementing unitarity constraints in meson spectroscopy is
very briefly outlined for Portuguese students of engineering sciences and
therefore non-experts in the field. After explaining the profound differences
between meson spectroscopy and atomic spectroscopy, attention is paid to the
shortcomings of standard Breit-Wigner parametrisations in the case of broad
and/or overlapping resonances. Finally, the manifestly unitary
Resonance-Spectrum-Expansion model, which lies at the heart of a recent invited
review paper by the present authors, is graphically presented, together with a
simple yet typical application to the long-controversial $K_0^\star(700)$
resonance.
