  Narrow high-mass states can arise despite large phase space when two nearly
degenerate states are coupled to the same dominant decay mode. Mixing via a
final-state interaction loop diagram can produce one very broad state and one
narrow state. Such a situation is generic in exotic hadrons where a color
singlet with given flavor and spin quantum numbers can be constructed with two
distinct internal color couplings of quarks. The simplest realization of this
idea are the (Q Qbar q qbar) tetraquarks containing two heavy and two light
quarks. We discuss possible experimental implications, including recent data
from Belle.
