National Internal Revenue Code (NIRC)
Section 132. Mode of Computing Contents of Cask or Package. - Every fractional part of a proof liter equal to or greater than a half liter in a cask or package containing more than one liter shall be taxed as a liter, and any smaller fractional part shall be exempt; but any package of spirits, the total content of which are less than a proof liter, shall be taxed as one liter.