Аннотация:A mathematical model is examined which can account for the different velocities obtained from optical and X-ray measurements of SNRs. In this model, the X-ray emission originates in hot gas behind a shock front created by the dispersion of a remnant, and the optical emission is caused by those parts of the shock front located at a given time inside density fluctuations, which are small clouds 5 to 10 times denser than the interstellar medium. It is shown that the velocity of the shock wave within a fluctuation determines the width of emission lines, while the average Doppler velocity of a filament (which is much less than the shock-wave velocity) depends on the asymmetry of a converging shock wave. Data for Pup A, the Cygnus Loop, IC 443, and Vel X are reviewed in the framework of this model.