The relative molar mass (symbol Mr) is a dimensionless quantity related to the molar mass M by Mr = M/(10–3kg·mol–1). In the case of an element, this reduces to the Atomic Weight Ar and, rounded to the nearest integer, it gives the number of nucleons in the most abundant isotope. The relative molar mass (often abbreviated RRM) is used most often in connection with large molecules such as polymers or polypeptides where it is used as a measure of chain length. Mr is sometimes called loosely the molecular weight; this is incorrect.