《周礼·冬官考工记·总叙》
因此说车有六等差数。凡观察车子的要领，必须从车子着地的部位开始，因此观察车子要从车轮开始。
车谓之六等之数；凡察车之道，必自载于地者始也，是故察车自轮始。
Therefore, it is said that chariots have six ranks. When observing a chariot, one must begin with the point where it touches the ground, so one must start with the wheels.