In one example, a document scanner having a document-receiving platen. The platen includes a plurality of alignment patterns disposed on the platen at known positions in a coordinate system. The scanner includes overlapping staggered scan bars disposed adjacent the platen, each scan bar fixed at a position to image a one-dimensional slice of a corresponding one of the alignment patterns. The scanner further includes a controller to determine, from the imaged slice, X and Y coordinates of each corresponding scan bar within the coordinate system.