<P> One fifth of a gallon was a common legal threshold for the difference between selling by the drink and selling by the bottle or at wholesale, and thus the difference between a drinking saloon or barroom and a dry - goods store . </P> <P> The fifth was the usual size of bottle for distilled beverages in the United States until 1980 . Other authorized units based on the fifth included ​ ⁄ pint and ​ ⁄ pint . </P> <P> During the 1970s, there was a push for metrication of U.S. government standards . In 1975, the Bureau of Alcohol, Tobacco and Firearms, in cooperation with the Distilled Spirits Council of the United States, proposed six metric - standard bottle sizes to take effect in January 1979 and these standards were incorporated into Title 27 of the Code of Federal Regulations . These sizes are 50, 100, 200, 375 (355 for cans), 500 (until June 1989), 750, 1000, and 1750 ml . </P>

Where does the term fifth of liquor come from