The side of a sheet or web of paper that has formed in contact with the papermaking machine's forming wire, as opposed to that formed on the top side of the paper, called the felt side. As water drains down through the wire from the papermaking furnish, small fibers (called fines), fillers, and sizing are lost through the mesh. Consequently, the wire side contains less fines, fillers, and sizing than the felt side. The paper's grain is also more pronounced on the wire side. Since the wire side contains less filler, it is somewhat stronger than the felt side, and is often the preferred side for printing. Paper that is to be folded also resists cracking better when the wire side is on the outside of a fold. Since it also has less fines, many of which remain as loose particles on the felt side, printing on the wire side results in less piling of paper debris on the blanket. However, the felt side of paper is preferred for other applications, especially for writing with a pen and ink, as feathering is minimal on the felt side. Letterheads are typically printed felt side up, and envelopes folded felt side out. A watermark will also read backwards from the wire side.
A paper's two-sidedness is primarily caused by using a traditional single-wire fourdrinier paper machine. The disparity between the two sides can be minimized by using a twin-wire former which sandwiches the papermaking furnish between two wires, imparting to the paper two wire sides. (See also Felt Side.)