  Due to the cluster reducibility of multiquark operators, a strong interplay
exists in tetraquarks between the compact structure, resulting from the direct
confining forces acting on quarks and gluons, and the molecular structure,
dominated by the mesonic clusters. This issue is studied within an effective
field theory approach, where the compact tetraquark is treated as an elementary
particle. The key ingredient of the analysis is provided by the primary
coupling constant of the compact tetraquark to the two mesonic clusters. Under
the influence of this coupling, an initially formed compact tetraquark bound
state evolves towards a new structure, where a molecular configuration is also
present. In the strong-coupling limit, the evolution may end with a shallow
bound state of the molecular type. The strong-coupling regime is also favored
by the large-$N_c$ properties of QCD. The interplay between compact and
molecular structures may provide a natural explanation of the existence of many
shallow bound states.
