英语中有很多使用身体部位，比如眼睛（eye）、耳朵（ear）、鼻子（nose）、嘴巴（mouth），甚至心脏（heart）等来构成短语，表达一定的意义。back作为名词的意思是“背，后背”，由此构成的短语有很多，今天我们学几个跟back有关的短语。

介词短语on the back是最常见的、简单的短语，它可以指“在背上”，比如：I can feel the warm sun on my back.我背上感受到了阳光的温暖。Please help me to sweep the dust on my back.请帮我掸一下我后背的尘土。

at one’s back不是“后背旁”，而是“支持某人；给某人撑腰”的意思。比如：He knows that he has the head of the Department at his back.他知道部门领导给他撑腰。He has a large number of people at his back.有很多人在支持他。

turn one’s back on的意思“不理；背弃”，on后面一般跟人称代词的宾格做宾语。比如：He’s always been kind to me. I can’t turn my back on him.他一直对我很好，我不能背弃他。She did not turn her back on her neighbours when they needed help.当邻居需要帮助时她不会不理的。

behind one’s back不是“在某人背后”的意思，而是指“背着某人，背地里（说坏话等）”。比如：This decision was taken behind my back.这个决定是背着我做出的。Don’t speak ill of others behind their backs.不要背地里说别人的坏话。They criticized him behind his back.他们在背后批评他。

pat on the back的意思是“鼓励；表扬”。比如：The players of the Olympic Game deserve a pat on the back.奥运会上的运动员们值得表扬。He deserves a pat on the back for all his hard work.他工作兢兢业业，值得嘉许。Dearest graduates, give yourself a pat on the back and treat yourself with a getaway! 亲爱的毕业生童鞋们，来一场毕业旅行犒劳自己吧！

back除了构成短语外，它还有很多我们常用到的复合名词，比如：backache背痛；backbone骨干，支柱；backbone network中枢网络，[计]骨干网；backbone system主干系统；backbone teacher骨干教师。backbiter造谣中伤者；backbreaker工作极度辛苦的人；backdoor后门等等。