The basic apothecaries system consists of the units pound, ounce, and scruple from the classical Roman weight system, together with the originally Greek drachm and a new subdivision of the scruple into either 20 ("barley") or 24 ("wheat") grains (). In some countries other units of the original system remained in use, for example in Spain the  and . In some cases the apothecaries and civil weight systems had the same ounces ("an ounce is an ounce"), but the civil pound consisted of 16 ounces.  is Latin for the seed of the carob tree.
Many attempts were made to reconstruct the exact mass of the Roman pound. One method for doing this consists in weighing old coins; another uses the fact that Roman weight units were derived from Roman units of length similar to the way the kilogramme was originally derived from the metre, i.e. by weighing a known volume of water. Nowadays the Roman pound is often given as 327.45 g, but one should keep in mind that (apart from the other uncertainties that come with such a reconstruction) the Roman weight standard is unlikely to have remained constant to such a precision over the centuries, and that the provinces often had somewhat inexact copies of the standard. The weight and subdivision of the pound in the Holy Roman Empire were reformed by Charlemagne, but in the Byzantine Empire it remained essentially the same. Since Byzantine coins circulated up to Scandinavia, the old Roman standard continued to be influential through the Middle Ages.