《明史·列传·卷一百零二》
京城距宣府、蓟镇只几百里，京营有九万兵卒，每年用一万兵卒戍守两镇，九年一个轮回，并不劳苦，而且使怯弱者同边防军一样强悍。
夫京师去宣府、蓟镇才数百里，京营九万卒，岁以一万戍二镇，九年而一周，未为苦也，而怯者与边兵同其劲矣。
The capital is only a few hundred miles away from the Xuanfu and Ji towns, and there are 90,000 soldiers in the imperial camp. Every year, 10,000 soldiers are sent to guard the two towns, and every nine years it is a cycle, which is not too hard, and it makes the cowards as strong as the frontier army.