An assembly in an exchange column includes a plurality of generally vertically adjacent layers of structured packing. Each layer includes a plurality of generally horizontally adjacent structured packing elements, each of which has an upper edge and a lower edge. Each layer has a top having a plurality of the upper edges and a bottom having a plurality of the lower edges. In a first layer of packing at least one of the upper edge and the lower edge is modified on at least one element. In a second layer of the packing, located vertically adjacent the first layer, the upper edges and the lower edges are unmodified on substantially all of the elements, whereby one of the unmodified upper edge and the unmodified lower edge on at least one of the elements in the second layer is adjacent one of the modified edges in the first layer.