  We study exact four-wave resonances among gravity water waves in a square box
with periodic boundary conditions. We show that these resonant quartets are
linked with each other by shared Fourier modes in such a way that they form
independent clusters. These clusters can be formed by two types of quartets:
(1) {\it angle-resonances} which cannot directly cascade energy but which can
redistribute it among the initially excited modes and (2) {\it
scale-resonances} which are much more rare but which are the only ones that can
transfer energy between different scales. We find such resonant quartets and
their clusters numerically on the set of 1000 x 1000 modes, classify and
quantify them and discuss consequences of the obtained cluster structure for
the wavefield evolution. Finite box effects and associated resonant interaction
among discrete wave modes appear to be important in most numerical and
laboratory experiments on the deep water gravity waves, and our work is aimed
at aiding the interpretation of the experimental and numerical data.
