Abstract: In a sheet stacking apparatus, in particular spiral slot stacker, one determines the relative position of a group of sheets 7A, 7B, for example distance ta between the sheets or total length tL of overlapping sheets. Sheet sensor 16 is mounted for this purpose at a large distance before input point 15 so that irregularities such as distance and/or overlap within the group of sheets 7A, 7B are taken into account and corresponding steps can be taken before leading sheet 7A is fed into stacker slot 2 of stacking wheel 1. Depending on the kind of irregularity ascertained, stacking wheel 1 is stopped, slowed down or accelerated to permit collision-free feed of the group of sheets into common slot 2 or into separate slots 2. In special embodiments one influences the sheet speed by means of separately controllable transport path segments 12a, 12b, and/or input point 15 by means of control finger 8.