  A classification of tetraquark states in terms of the spin-flavour, colour
and spatial degrees of freedom has been constructed. The permutational symmetry
properties of both the spin-flavour and orbital parts of the quark-quark and
antiquark-antiquark subsystems are briefly discussed. This classification is
useful both for model-builders and experimentalists. An evaluation of the
tetraquark spectrum is obtained from a Iachello mass formula for tetraquark
systems. The ground state tetraquark nonet is identified with $f_{0}(600)$,
$\kappa(800)$, $f_{0}(980)$, $a_{0}(980)$. The mass splittings predicted by
this mass formula are in agreement with the KLOE, Fermilab E791 and BES
experimental data.
