The steel used in the Chinese swords, called chi-kang (combined steel), was similar to pattern welding, and edges of it were often forge welded to a back of soft iron, or jou thieh. In trying to copy the Chinese method, the ancient smiths paid much attention to the various properties of steel, and worked to combine them to produce an internal macro-structure that would provide a similar combination of hardness and toughness. Like all trial-and-error, each swordsmith often attempted to produce an internal structure that was superior to swords of their predecessors, or even ones that were better than their own previous designs.[31] The harder metals provided strength, like "bones" within the steel, whereas the softer metal provided ductility, allowing the swords to bend before breaking.