Apothecaries' system

The apothecaries' system of mass is an obsolete system formerly used by apothecaries (now called pharmacists or chemists) in English-speaking countries. The system is related to the English avoirdupois and troy systems, as they use the same mass for a grain. Sometimes "ap" is added to the front of the unit to identify it as part of the apothecaries' system (the abbreviation for avoirdupois is "av"). Similar systems had been in use in other European countries.

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During the first half of the 20th century, the apothecaries' system was replaced by the metric system. In the United States, it is still occasionally used, for example with prescribed medicine being sold in six ounce (℥ vi) bottles. An old maxim related to the problem involved in apothecary weight calculations when converting from avoirdupois weight — a grain is a grain is a grain — the pound weight in each system being different. The apothecary would buy the drugs by avoirdupois and compound and dispense by apothecary weight. Another anomaly, when converting grains to metric weight, 60 mg was considered the same as 64 mg or 65 mg = 1 gr.

After the introduction of the imperial gallon (1824), some English apothecaries started using a system of volume that was closely related to the system of mass, taking advantage of the approximate equivalence of weight and volume of water and similar fluids with equal-named units. It was abolished effective 1 January, 1971 after falling out of use.