The thickness of the "stratum corneum" varies throughout the body. In the palms of the hands and the soles of the feet (sometimes knees, elbows, knuckles,) this layer is stabilized and built by the "stratum lucidum" (clear phase) which allows the cells to concentrate keratin and toughen them before they rise into a typically thicker, more cohesive SC. The mechanical stress of heavy structural strain causes this SL phase in these regions which require additional protection in order to grasp objects, resist abrasion or impact, and avoid injury. In general, the "stratum corneum" contains 15 to 20 layers of corneocytes. The stratum corneum has a thickness between 10 and 40 μm.