Note: The level of the tensile modulus of carbon fibres is controlled by the degree of preferred orientation of the layer planes in the direction parallel to the fibre axis. $C_{11}$, the elastic constant of graphite single crystals in the direction of the layer planes, is $1060\pm 20\text{GPa}$. In general, the ratio of tensile strength to tensile modulus is smaller than $1\times {10}^{-2}$ for carbon fibres type HM (but the tensile strength is influenced by flaws in the fibres and may be improved in the future). Carbon fibres type UHM (ultra-high modulus) have moduli of elasticity in excess of $600\text{GPa}$, surpassing 50% of the theoretical $C_{11}$ number. Such high values of Young's modulus can be achieved most readily in mesophase pitch-based carbon fibres (MPP-based carbon fibres).