A constant designated E, the ratio of stress to corresponding strain when the material behaves elastically. Young's modulus is represented by the slope E = ΔS/Δε of the initial straight segment of the stress-strain diagram. More correctly, E may be represented as the slope of the tangent or the slope of the secant connecting two points of the stress-strain curve. The modulus is then designated as tangent modulus or secant modulus at stated values of stress. The modulus of elasticity applying specifically to tension is called Young's modulus. Many materials have the same value in compression.