Flax fibres occur in the stem of the plant Linum usitatissimum occurring as some 30 bundles around the woody core. The word flax comes from the Old English term 'fleax'. Flax describes the fibre whilst 'linen' is used to reffer to the yarn spun from this fibre and to the cloth woven from this yarn. Each flax bundle consists of 10 to 14 separate fibres.

As with cotton, cellobiose is also the monomer unit for flax. The degree of polymerisation is typically 18,000 cellobiose units in flax, compared with 5,000 in cotton. The flax polymer is the longest known linear textile polymer. The polymer is highly crystaline due to the long polymer length and as a consequence the fibre is extremely duarbale with a high tensile strength. Linen fabrics wrinkle more easily than cotton fabrics due to the higher crystallinity which imparts a very inelastic quality to the fibres. Flax will resist being flexed or bent giving linen its stiff handle. Wrinkling and creasing is explained by virtue of the highly crystaline polymer system fracturing when the fibres are distorted.