一圍之木，持千鈞之屋；五寸之鍵，而制開闔。
A tree one arm-span around can hold a one-thousand-jun structure in place; a five-inch bar can control the opening of a door.
豈材足任哉，蓋所居要也。
It is not that these objects are in themselves capable of such feats, but rather that they each occupy a crucial position.