Materials such as concrete or prestressing wire have a variable Young's modulus (E) so that the particular value of E adopted must be either the slope of the tangent to the stressstrain curve or that of the secant. The latter is the line that joins the origin of the curve to, for instance, the 0.1% proof stress, expressed on the curve. For a material within its elastic range, the secant will coincide with the tangent.