In 1970 Geoffrey Bibby drew attention to a tablet from Ur of c. 1800 B.C. which equates weights on the Dilmun weight standard with those on the standard of Ur. He concluded that the Dilmun weight standard was the same as that used in the Indus Valley, but was puzzled by the inconsistencies in the arithmetic, which he explained by assuming that the merchants took a commission each time the weight was converted from one standard to another, that is a different rate of exchange was used to convert from Dilmun weights than was used to convert from Ur weights. Collation of the tablet in the Iraq Museum, however, has removed the problems in the mathematics and the need for Bibby's elaborate commercial procedures.