  The detailed way in which duality between sum of exclusive states and the
free quark model description operates in semileptonic total decay widths, is
analysed. It is made very explicit by the use of the non relativistic harmonic
oscillator quark model in the SV limit, and a simple interaction current with
the lepton pair. In particular, the Voloshin sum rule is found to eliminate the
mismatches of order $\delta m/m_b^2$.
