The interplanetary perturbations associated with Forbush decreases (FDs) during the period 1965–1974 are studied. The systematic appearance of the results obtained confirms once again that type-IV solar flares are the origin of these perturbations. The front perturbation is a wide (∼100° in longitude) driven shock followed by a magnetic blob and a high-speed plasma cloud of about 0.5 A.U. average radial dimension at the Earth orbit, emitted in a short time interval (<15 h) immediately following the beginning of the type-IV burst. The FD-modulated region is found to be included between two boundary streams which rotate with the Sun; therefore the FD can be observed also in the interplanetary regions which were not swept by the front perturbation. The FD amplitude appears to be correlated with an empiric parameter which caters for the importance of the shock and magnetic blob at the front perturbation.