This International Standard specifies two permeametric methods for the determination of the fineness of flax fibres : — a reference method, with two compressions, using a test piece of parallel fibres (clause 5); — a simplified method, with one compression, using a test piece of fibres distributed "at random" (clause 6). These methods apply to the various forms possible for flax fibres, i.e. long strands, broken strands, all kinds of tow and at all stages of manufacture of these substances.