Substrate for a security document

A substrate 30 for a security document comprises a substantially planar polymer sheet or laminate having a tensile strength which differs in at least two directions in the plane of the substrate. Part of the substrate has an elongate deformation extending substantially orthogonally to the direction exhibiting the greater tensile strength. The elongate feature may be a raised feature or may be embossed into the substrate. The greater tensile strength is preferably at least 100N/mm2. The polymer is preferably one of polypropylene, polyethylene terephthalate, nylon or polyethylene. One or both sides of the substrate may be provided with a print receptive opacifying coating. The security device may be a transferable label on a carrier. The substrate may be provided with a first array of lines wherein the lines comprise materials which appear different from each other and a second surface relief array of lines imposed on the first array.