The "neutral line" (also called the "neutral axis") is an imaginary line that can be drawn through a cross-section of the workpiece that represents the locus where no tensile nor compressive stresses are present on the work. Its location in the material is a function of the forces used to form the part and the material yield and tensile strengths. In the bend region, the material between the neutral line and the "inside" radius will be under "compression" during the bend. The material between the neutral line and the "outside" radius will be under "tension" during the bend. For small radii, the line with no tension/compression no longer coincides with the line with zero "strain". Furthermore, the bend allowance (see below) in air bending depends on the shoulder distance of the die. As a result, the bending process is more complicated than it appears to be at first sight.