Angel’s share: The portion of Bourbon in an aging barrel that’s lost to evaporation.

Backset: The thin, watery part of a previously distilled batch of whiskey mash that is added ‒ or “set back” ‒ into the next batch. Also “sour mash, setback, stillage or spent beer.”

Barrel proof: Whiskey bottled at the desired proof while aging in the barrel. No water is added before bottling, so these Bourbons are higher proof than others.

Beer still: A giant apparatus in which the main component is a very tall metal column used to separate the alcohol from the water in the distiller’s beer by vaporizing the alcohol content. Also called a “continuous still.” The spirit produced is called “low wines.”

Bourbon (straight): A whiskey made from a mash containing at least 51 percent corn, distilled out at a maximum of 160° proof, aged at no more than 125° proof for a minimum of two years in new charred oak barrels. If the whiskey is aged for less than four years, its age must be stated on the bottle. No coloring or flavoring may be added to any straight whiskey.

Bung: The stopper used to seal a barrel.

Charring: The process that sets fire to the interior of barrels for less than one minute and creates a layer of charred wood. Distillers can choose from four levels of char.

Corn whiskey: A whiskey made from a mash containing a minimum of 80 percent corn and, if it is aged at all, must be aged in used or uncharred oak barrels.

Distiller’s beer: The thick, fermented mash of cooked grains, water and yeast that is transferred from the fermenter to the beer still for the first distillation.

Doubler: A large copper still used to accomplish the second distillation of American whiskey. It effectively removes impurities and concentrates the alcohol even further. “Low wines” go in; “high wines” come out.

Fermentation: The process by which yeast transforms sugar into alcohol and carbon dioxide.

Fermenter: A giant tub made of metal or cypress in which the mash of cooked grains and water meet the yeast. They mingle, the yeast begins to act on sugars in the grain, and fermentation occurs over a few days. This produces alcohol within the mash and turns it into distiller’s beer.

Heads: The first section of the high wines to exit the doubler or thumper; this spirit is high in impurities and sent back to the still for redistillation.

High wines: The final spirit produced by the secondary distillation, ready for aging.

Low wines: The name of the spirit after it has passed through the beer or continuous still for its first distillation.

Malted barley: Barley that has been partially germinated and then heated or roasted to stop the germination. Malted barley (or any malted grain) contains enzymes that convert starches into the fermentable sugars on which yeast feeds. These enzymes are not present in unmalted grains.

Mash: The mixture of cooked grains and water before the yeast is added to start fermentation.

Mashbill: The grain recipe used to make whiskey.

Mash tub: A large tub where the grains are combined with water and cooked to soften them and break down the starch into simple sugars before the resulting “mash” is transferred to the fermenter. Also called a “cooker.”

Nose: The aroma of a whiskey.

Proof: Measurement of beverage alcohol on a scale, in America, of 200. A 100° proof spirit contains 50 percent alcohol.

Rackhouse: The building in which whiskey is aged, sometimes referred to as the “warehouse.”

Ricks: The wooden structures on which barrels of whiskey rest during aging.

Rye whiskey (straight): A whiskey made from a mash containing at least 51 percent rye, distilled out at a maximum of 160° proof, aged at no more than 125° proof for a minimum of two years in new charred oak barrels. If the whiskey is aged for less than four years, its age must be stated on the bottle. No coloring or flavoring may be added to any straight whiskey.

Single barrel whiskey: Whiskey drawn from one barrel that has not been mingled with any other whiskeys.

Small batch whiskey: A product of mingling select barrels of whiskey that have matured into a specific style.

Tails: The last section of high wines to exit the doubler or thumper; this spirit is high in impurities and sent back to the still for redistillation.

Thief: A tubular instrument for removing a sample from a barrel.

Thumper: One of the types of stills used to accomplish the second distillation of American whiskey. It effectively removes impurities and concentrates the alcohol even further. “Low wines” go in; “high wines” come out. Thumpers differ from doublers in that the low wines enter a thumper as vapors that are bubbled through water, causing the stills to make a thumping sound; a doubler makes no distinctive noise since the low wines enter in condensed, liquid form.

Wheated bourbon: Bourbon made from a mashbill that contains wheat instead of rye grain.

Yeast: A living organism that feeds on fermentable sugars, transforming them to beverage alcohol, congeners, carbon dioxide, and heat.