הלכה על שקלים 1:2

From the laws of the commandment is that which they, may their memory be blessed, said (Arakhin 18b) that we received from the tradition that these years that are stated for appraisals are measured day to day - meaning from the day of birth - and also that all the shekel-coins stated there are the holy shekels. And we have known from the received tradition that the weight of a holy shekel is three hundred and twenty barley grains of pure silver. And the Sages have already added upon it and made its weight like the weight of the coin called the sela at the time of the Second [Temple], which is three hundred and eighty-four medium barley grains (so is it in Mishneh Torah, Laws of Sheqel Dues 1:2-3). And they, may their memory be blessed, said that this sela is (four) dinar, and the dinar is six maah - and the maah is what was called gerah in the days of Moshe, as Onkelos translates gerah as maah - and its weight is sixteen barley grains.