The 208th mitzvah is that we are commanded to have accurate weights, scales, and measures, and to insure that they are exact.

The source of this commandment is G‑d's statement1 (exalted be He), "You must have an honest balance, honest weights, an honest eifa, and an honest hin."

In the words of the Sifra: "The phrase 'an honest balance' means that you must insure that the balances are totally accurate. 'Honest weights' means that you must insure that the weights are totally accurate. 'An honest eifa' means that you must insure that all eifas are totally accurate. 'An honest hin' means that you must insure that all hins are totally accurate." You are already aware that an eifa is a dry measure and a hin is a liquid measure.

Although2 the actual type of measure varies, they serve a single function, since what is weighed or measured is just a particular quantity of something. All these categories, i.e. scales, weights, and dry and liquid measures are collectively called middos. So too, the commandment to insure that each corresponds exactly to the commonly accepted amount is called mitzvas middos.

In the words of the Sifra: "On this condition I brought you out of Egypt — on condition that you accept upon yourselves mitzvas middos; because whoever acknowledges mitzvas middos acknowledges the redemption from Egypt and whoever denies mitzvas middos denies the redemption from Egypt."

The details of this mitzvah are explained in the 5th chapter of tractate Bava Basra.3