  Despite the apparent simplicity of meson spectroscopy, light scalar mesons
cannot be accommodated in the usual $q\bar q$ structure. We study the
description of the scalar mesons below 2 GeV in terms of the mixing of a chiral
nonet of tetraquarks with conventional $q\bar q$ states. A strong
diquark-antidiquark component is found for several states. The consideration of
a glueball as dictated by quenched lattice QCD drives a coherent picture of the
isoscalar mesons.
