During printing, the softer texture of the felt side of an uncoated paper may pick up slightly more ink than the wire side of the same sheet, and the printer may have to adjust ink densities to compensate for this. (Quickprint)

There are several known causes of inherent curl in paper. For example, the wire side and felt side of a sheet exhibit different degrees of fibre orientation, with wire side fibres tending to be more aligned in the machine direction than felt side fibres. (PaperlinkX)

Paper made on a twin-wire fourdrinier machine has either two felt sides or two wire sides; this is because two webs of pulp are laid down simultaneously and pressed together as the paper is dried and finished. (Net Industries and its Licensors)