《宋史·志·卷二》
一叫天铁，是天的军备，所以在紫微垣左右，用来抵御国难。
一曰天钺，天之武备也，故在紫微宫左右，所以御难也。
One is called Tiantie, which is the military equipment of heaven, so it is located on the left and right of Ziweiyuan to resist national calamities.