Why is whiskey sold in fifths?

The term "fifth" in the context of alcohol refers to the size of the bottle, rather than the amount of liquid it contains. It got its name because it is equivalent to one-fifth of a US gallon.

The fifth became a popular size for whiskey bottles in the 19th century, as it was a convenient size for taverns and saloons to serve their customers. It was also a popular size for transporting whiskey, as it was easy to handle and store.

The fifth has remained the standard size for whiskey bottles in the United States to this day, although other sizes are also available, such as pints, quarts, and fifteens (equivalent to one-fifteenth of a gallon).