Chris Reed, age 13, of Portland, Mich., for his question:

WHY IS LB. THE ABBREVIATION FOR POUND?

We abbreviate some words by shortening them. For example in. is written for the word inch. Some words are abbreviated by omission and so we write, simply a. for the word acre. Other words are abbreviated by contraction: hr. is used for hour.

There  are some words where the abbreviated form is actually a substitution. The abbreviation lb. for the word pound is such a case. Lb. is the abbreviated form of the word libra. In Latin, libra means balance or weight, and in the ancient Roman language, it actually meant the pound measure. So it seems perfectly proper to substitute the abbreviation of the word libra for the word pound.