This method describes a horizontal plane procedure for the determination of the coefficient of static and kinetic friction of paper measured when sliding against itself. Although the method is basic in concept, the precision statement has been developed on uncoated writing and printing papers. While the use of the method is recommended for those grades only, it may be used with other types of paper given specific attention to special paper characteristics and with the understanding that the precision and accuracy may not be the same. The horizontal instrument requires some means of movement of the specimen in relation to the surface upon which it rests. The coefficient of friction (COF) is measured directly from the resistance to tangential motion and the applied weight pressing two pieces of paper together. Static COF relates to the force required to initiate movement between two surfaces while kinetic COF relates to the force required to cause continu-ation of the movement at uniform speed. An inclined plane method is described in test method T 548, which gives similar results for static COF but it cannot be used for the determination of kinetic COF. The choice of approach depends on the equipment available and the means of measurement. The determination of COF for packaging materials are described in T 503, T542, T 815, and T 816. The methods differ in that in testing packaging materials, the two specimens are allowed to slip upon one another three times before the force measurement is determined, while in this method the determination is made on the first slip.