  A general 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 discussed in short. This
classification is model indipendent and useful both for model-builders and
experimentalists. An evaluation of the tetraquark spectrum is obtained from a
generalization of a Iachello mass formula, originally developed for the $q\bar
q$ mesons. The ground state tetraquark nonet is identified with $f_{0}(600)$,
$\kappa(800)$, $f_{0}(980)$, $a_{0}(980)$.
