Bourbon whiskey distillers must follow government standards for production. By law (), any "straight" bourbon must be: produced in the United States; made of a grain mix of at least 51% corn; distilled at no higher than 160 proof (80% ABV); free of any additives (except water to reduce proof for aging and bottling); aged in new, charred white oak barrels; entered into the aging barrels at no higher than 125 proof (62.5% ABV), aged for a minimum of 2 years, and bottled at no less than 80 proof (40% ABV).