Qin Shi Huangdi, the first Chinese emperor, who united the country, made strict new laws, reformed the coinage and burned all books he disagreed with. He ruled from 221 to 207 BC, and was buried with 6,000 terracotta warriors guarding his tomb. He wanted to stop carts crashing on rutted roads, so gave orders that they should all have wheels the same distance apart. That way, they could follow the same track.