the key of the north gate—places of strategic importance in the north ❍ 杞子自郑使告于泰，曰： “郑人使我掌其北门之管，若潜师以来，国可得也。” (《左传·僖三十二年》)Qi Zi had sent information from Zheng to Qin,saying，“The people of Zheng have entrusted to my charge the key of their north gate. If an army come secretly upon it,the city may be got.”