也可以说，上午，下午!~~

医学上有上吐,下泻

There are a lot of Chinese words used "上A" "下A" structure! Don't worry about it!.. At first, you have to know the first meaning of "上" and "下": "上" means "up" and "下" means "down", so when "上,下" is a verb, you can think the meaning of it is "go up or go down". For example: 上車(get on), 下車 (get off), 上飛機 (to board an airplane) 下飛機 (to get off a plane), 上山 (to go up the mountain) 下山 (to go downhill)... But the meaning of "上, 下" in 上班，下班，上课，下课 is not "up, down" . It means "to go to, to get off". When someone ask you: "what are you doing?" or "What are you going to do?" , you can answer "我要上班了!" or "我在上班" ... The last one I want to tell you that, in any case, 上 and 下 always are opposite! Best regards,

我觉得这是误导。。 就不是一个词性的 不过我也不知道怎么说 感觉跟量词一样难解释 sigh

上山，下山 上船，下船 上飞机，下飞机 上卷，下卷 或者 上冊，下冊 上半年，下半年 上线，下线 （go online， go offline） 上级，下级 (The superior, the subordinate) 上文，下文