This paper presents an exploratory study investigating how physical and digital documents are used in combination on tabletops. Our results identify hybrid piles as the most common grouping concept and show that users willingly occlude digital documents with physical paper. These findings have considerable impact on the design of novel hybrid interaction techniques, which we sketch at the end of this paper. Keywords Interactive surface, paper, occlusion, study, tabletop. ACM Classification Keywords H.5.2 User Interfaces General Terms Human Factors