The earliest Dao date from the Shang Dynasty in China's Bronze Age, and are known as zhibeidao (直背刀) – straight backed knives. As the name implies, these were straight-bladed or slightly curved weapons with a single edge. Originally bronze, these weapons were made of iron or steel by the time of the late Warring States period as metallurgical knowledge became sufficiently advanced to control the carbon content. Originally less common as a military weapon than the jian – the straight, double-edged blade of China – the dao became popular with cavalry during the Han dynasty due to its sturdiness, superiority as a chopping weapon, and relative ease of use – it was generally said that it takes a week to attain competence with a dao/saber, a month to attain competence with a qiang/spear, and a year to attain competence with a jian/straight sword.