  We propose an effective field theory to describe hadrons with two heavy
quarks without any assumption on the typical distance between the heavy quarks
with respect to the typical hadronic scale. The construction is based on
Non-Relativistic QCD and inspired in the strong coupling regime of Potential
Non-Relativistic QCD. We construct the effective theory at leading and
next-to-leading order in the inverse heavy-quark mass expansion for arbitrary
quantum numbers of the light degrees of freedom. Hence our results hold for
hybrids, tetraquarks, double heavy baryons and pentaquarks, for which we also
present the corresponding operators at Non-Relativistic level. At leading
order, the effective theory enjoys heavy quark spin symmetry and corresponds to
the Born-Oppenheimer approximation. At next-to-leading order, spin and velocity
dependent terms arise, which produce splittings in the heavy-quark spin
symmetry multiplets. A concrete application to double heavy baryons is
presented in an accompanying paper.
