1. 当by表示“按；论”的意义时，单位词hour, day, month, year, dozen, gallon, ton, yard等前要用定冠词。例如：

They let out cars by the day. 他们按天出租汽车。

The cloth is sold by the yard. 这种布是按码卖的。

2. 当by表示“按；论”的意义时，表示总称的单位词（重量、高度、深度、大小、体积、容量、价值……）通常不用冠词。例如：

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

Sort these apples by size. 将这些苹果按大小分好。

By value, 35% of euro notes in circulation are in the highest denomination, the 500 bill that few people ever see. 按价值计算，流通的欧元纸币当中有35%都是很少有人见过的最高面值纸币，即500欧元纸币。