唇齿相依 lips and teeth mutually dependent
Explanation
比喻关系密切，互相依靠，像嘴唇和牙齿那样，缺一不可。
It means that the relationship is close and interdependent, like lips and teeth, which are indispensable to each other.
Origin Story
三国时期，蜀汉丞相诸葛亮临终前，曾告诫后主刘禅要与东吴保持联盟，因为蜀吴两国唇齿相依，共同对抗曹魏，才能保全蜀汉的基业。诸葛亮深知，如果蜀汉与东吴关系破裂，则将面临魏国的强大压力，最终可能导致灭亡。诸葛亮的遗言，体现了蜀国与东吴之间的特殊关系，他们相互依靠，共同抵御外敌，就像嘴唇和牙齿一样，密不可分。
During the Three Kingdoms period, Zhuge Liang, the Prime Minister of Shu Han, warned the successor Liu Chan before his death to maintain an alliance with Eastern Wu, because Shu and Wu were closely dependent on each other to fight against Cao Wei together in order to preserve the foundation of Shu Han. Zhuge Liang knew very well that if Shu Han's relationship with Eastern Wu broke down, it would face enormous pressure from the Wei state, and could eventually lead to its demise. Zhuge Liang's last words reflected the special relationship between Shu and Eastern Wu, who relied on each other and resisted external enemies together, inseparable like lips and teeth.
Usage
形容关系密切，互相依赖。常用于比喻国家、地区或个人之间的关系。
Describes a close and interdependent relationship. Often used to illustrate the relationship between countries, regions or individuals.
Examples
-
唇齿相依的两个国家，应该互相帮助，共同发展。
chún chǐ xiāng yī de liǎng gè guójiā, yīnggāi hù xiāng bāngzhù, gòngtóng fāzhǎn.
Two countries that are closely related should help each other and develop together.
-
我们两人唇齿相依，患难与共，共同走过了人生的风风雨雨。
wǒmen liǎng rén chún chǐ xiāng yī, huàn nàn yǔ gòng, gòngtóng zǒuguò le rénshēng de fēngfēng yǔyǔ
We two are closely related and have gone through the ups and downs of life together.