In order to eliminate stress within the material, sheets must be leveled. The fundamental principle of roller leveling is to selectively elongate different sections of the material. Below the material's yield point, the shorter "fibers" of the sheet are proportionally stretched to achieve a consistent "fiber" length. This is accomplished by passing the material through the leveling machine where it undergoes repeated cyclic up-and-down bending at a small radius of curvature, thereby elongating the shorter material "fibers." As the sheet moves toward the exit of the leveling machine, the amplitude of the up-and-down bending gradually decreases, eliminating the material deformation caused by the intense bending at the entry of the machine. When all the "fibers" in the material are nearly the same length, the sheet becomes flat — or, in other words, "leveled."