by 表示单位时，意思是“以……计”，其后通常接表示计量单位的单数名词，且名词前要用定冠词。如：

They are paid by the hour (by the month). 他们所得报酬按小时（月）计。

Milk is sold by the pint, butter by the pound, and eggs by the dozen. 牛奶按品脱卖，黄油论磅卖，蛋类论打卖。

但如果是接抽象名词，则不用冠词。如：

Bananas are usually sold by weight. 香蕉通常按重量卖。