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Leather is prepared from vertebrate skin by the chemical stabilization of the fibrous protein collagen, the main solid constituent of skin. The natural three-dimensional fibrous weave of the collagen fibrils is retained intact in the leather, and this paper describes in detail the relation between the fibrous weave as seen under the light and scanning electron microscopes and the physical properties of the leather. In the main, skins of cattle, sheep and goats are used. These differ in total thickness, fibre bundle size and weave pattern, but offer a variety of raw material from which the tanner can select the skin type best suited for a particular end use. The tanning process can also modify the natural weave in order to achieve the required physical properties. For example the processing can allow fine spaces to remain between the fibrils, so that they are free to move over each other within the fibre bundle. Such a bundle will be highly flexible as will be the leather as a whole. Conversely, if fibrils adhere to each other and are not free to move the leather will be firm. Such fine spaces can be recognized under the microscope as longitudinal striations. Another important feature which is influenced by the tanning process is the angle at which the fibres interweave in relation to the grain surface. A low angle of weave is required for high tensile strength as this allows the pull to be transferred along the fibre axis. A low angle of weave also allows more frequent interweaving of the fibres within a given thickness of leather, than if the angle is high. As frequent interweaving of the fibres is a prerequisite for strength the tear strength of leather is highly dependent on the angle of weave. This is particularly so in certain types of leather prepared from cattle skin, which is cut into layers to obtain the required leather thickness. For leather to accommodate stretching, such as that occurring when lasting shoe uppers, or compression and creasing, the individual fibrils need to be free to move within the bundle, and the bundle within the weave as a whole. The changes in the fibre structure that occur under such conditions can be followed microscopically and are described in detail in this paper.