The welt, a strip of leather, linen or synthetic sewn to the inner edge and upper insole. In addition to the welt, the stitching holds the material together. When the welt strip has formed a cavity, it is then filled with cork. This is a lightweight, insulating, and waterproof material that most importantly lets your feet breathe. A shoe is finished off when the sole is sewn to the upper. This process is done by stitching and sticking using hide glue or contact adhesive.