  For very heavy quarks, relations derived from heavy-quark symmetry imply
novel narrow doubly heavy tetraquark states containing two heavy quarks and two
light antiquarks. We predict that double-beauty states will be stable against
strong decays, whereas the double-charm states and mixed beauty+charm states
will dissociate into pairs of heavy-light mesons. Observing a new double-beauty
state through its weak decays would establish the existence of tetraquarks and
illuminate the role of heavy color-antitriplet diquarks as hadron constituents.
