Bourbon may dominate the conversation, but American whiskey includes rye, wheat, malt, corn and other types. Their identity begins with the percentage of a named grain in the mash and continues through distillation strength and maturation rules.
Bourbon
Bourbon uses at least 51% corn, is distilled at no more than 160 proof and enters new charred oak at no more than 125 proof. It must be made in the United States, but it is not restricted to Kentucky.
Rye and wheat whiskey
Rye whiskey uses at least 51% rye; wheat whiskey uses at least 51% wheat. The grain threshold does not prescribe flavour, but rye often brings an assertive herbal or spicy impression while wheat can seem rounder. Fermentation, distillation and oak can change those expectations.
Straight whiskey
“Straight” adds an ageing requirement of at least two years and limits additions. When straight whiskey is younger than four years, its label generally needs an age statement. Bottled-in-bond is a separate regulated designation with its own production season, distillery, warehouse and proof requirements.
Proof
US proof is twice alcohol by volume, so 100 proof equals 50% ABV. Entry proof, barrel proof and bottling proof describe different moments. Higher bottling proof is not automatically better; it changes concentration and how the whiskey responds to water.
Primary reference: US Alcohol and Tobacco Tax and Trade Bureau.