Commentary for Deuteronomy 25:13

אבן ואבן [THOU SHALT NOT HAVE IN THY BAG] DIVERSE STONES — i.e., weights.

גדולה וקטנה, the vendor make himself two weights each marked as representing the same measure, but one being smaller than the other. When using both simultaneously, i.e. filling each next to the other, the total quantity is correct, but one represents more than half, the other less. It is therefore easy to deceive a customer who has once observed that when the two are used together they give the correct measure. This is why the Torah demands that “each” measure must be true in quantity of weight, i.e. אבן שלמה, “a true weight, etc.,” instead of “true weights.”

לא יהיה בכיסך אבן ואבן, “You must not have in your pocket two stones purporting to weigh the same but designed to deceive the customer.” Ibn Ezra explains that the word בכיסך is a reference to the pouch in which weights are kept. The word is derived from כסס, and is used in the sense of uniform weights in Proverbs 1,14 כיס אחד יהיה לכולנו, “let us all have a common purse.” It also appears in a slightly different form in Exodus 12,4 where it refers to the number of people arranging to receive equal shares of a Paschal lamb.