This is the kind of thing that makes the tax code so long:

Any natural grape wine may be sweetened after fermentation and before tax payment with pure dry sugar or liquid sugar if the total solids content of the finished wine does not exceed 12% of the weight of the wine and the alcoholic content of the finished wine after sweetening is not more than 14% by volume; except that the use under this subsection of liquid sugar shall be limited so that the resultant volume will not exceed the volume which could result from the maximum authorized use of pure dry sugar only.

~ excerpt from 26 USC § 5383