  A complete classification of tetraquark states in terms of the spin-flavor,
color and spatial degrees of freedom was constructed. The permutational
symmetry properties of both the spin-flavor and orbital parts of the
quark-quark and antiquark-antiquark subsystems are discussed. This complete
classification is general and model-independent, and is useful both for
model-builders and experimentalists. The total wave functions are also
explicitly constructed in the hypothesis of ideal mixing; this basis for
tetraquark states will enable the eigenvalue problem to be solved for a
definite dynamical model. This is also valid for diquark-antidiquark models,
for which the basis is a subset of the one we have constructed. An evaluation
of the tetraquark spectrum is obtained from the Iachello mass formula for
normal mesons, here generalized to tetraquark systems. This mass formula is a
generalizazion of the Gell-Mann Okubo mass formula, whose coefficients have
been upgraded by means of the latest PDG data.
  The ground state tetraquark nonet was identified with $f_{0}(600)$,
$\kappa(800)$, $f_{0}(980)$, $a_{0}(980)$. The mass splittings predicted by
this mass formula are compared to the KLOE, Fermilab E791 and BES experimental
data. The diquark-antidiquark limit was also studied.
