We give an explanation for the onset of fluid-flow-induced flutter
in a flag. Our theory accounts for the various physical mechanisms
at work: the finite length and the small but finite bending stiffness
of the flag, the unsteadiness of the flow, the added mass effect,
and vortex shedding from the trailing edge. Our analysis allows us
to predict a critical speed for the onset of flapping as well as the
frequency of flapping. We find that in a particular limit corresponding to a low-density fluid flowing over a soft high-density flag, the
flapping instability is akin to a resonance between the mode of
oscillation of a rigid pivoted airfoil in a flow and a hinged-free
elastic plate vibrating in its lowest mode.