"The barrel is the master distiller's accomplice — it does sixty to seventy percent of the work."
Bourbon is not just a flavor — it's a legal designation. Six federal regulations define what can call itself bourbon. Break any one of them and you're making whiskey, not bourbon.
Bourbon must be produced in the United States. While most comes from Kentucky, bourbon can legally be made in any state — Texas, New York, Tennessee (if it doesn't charcoal-filter).
The mash — the mix of grains cooked and fermented — must be at least 51% corn. Most bourbons run 65–80%. Corn gives bourbon its characteristic sweetness and body.
The distillate must not exceed 160 proof (80% ABV). Higher proofs strip out the grain character that defines bourbon. This is why bourbon must be distilled in column stills, not continuous stills that push proof higher.
The single most important rule. Bourbon must age in new, never-before-used charred oak barrels. This is what gives bourbon its color, vanilla, caramel, and oak character — and why Scotch distillers happily buy used bourbon barrels.
The spirit enters the barrel at no more than 125 proof (62.5% ABV). Lower entry proof means more water relative to alcohol, and more contact between the spirit and the wood, producing richer, more complex flavors.
Bourbon must be bottled at a minimum of 80 proof (40% ABV). It can be diluted with pure water down to 80 proof, or bottled "cask strength" at whatever proof it left the barrel — sometimes over 130 proof.