Pattern welding is the practice in sword and knife making to hammer out the metal, fold it over, and weld the metal piece back onto itself. Pattern welding was originally a way of reducing slag and impurities from the metal and to homogenise the often erratic carbon content of the iron and steel yielded by early metallurgy. Pattern welding is named so because a blade forged in this manner often display bands of slightly different colouration along its entire length. These bands can be brought out with for example acid etching for cosmetic purpouses.
Pattern welding is widely used today in custom knife making, as a cosmetic enhancement.