In addition to conventions, it is also important that idioms are already an established combination, and their meaning and even usage may change with the changes of the times (such as unfounded), but you can't change this combination alone, you need to give specific context in the combination. The meaning and combination of idioms are certain, and they can exist independently and be correctly understood without content.
Extended data:
Source: Songshi Puji's "Five Lights Meeting Yuan" Volume 11: "Q:' When I come, I have no time and space, and I am fascinated by two ways. How can you not be fascinated? "The teacher said,' The head of the scale is half a catty, and the tail is eight taels.' "
Example:
1, Songshi Baiwei's "Zhong Jian Guo Jing Continued Lamp Record": "A scale is as hard as iron, and eight liang turned out to be half a catty."
2. Song Anonymous "Zhang Xie Champion" Play 28: "Two pounds and a half, don't be angry when you go home."
3. Shi Ming Nai 'an's "Water Margin" is back to the 107th: "People will see that he has two skills, both of which are almost the same, and his clothes are similar."
References:
Baidu encyclopedia-half a catty and 82 liang