Cask strength, barrel strength, proof. What do all these things mean? Excellent question. Proof is a measurement of alcohol by volume in America. It's generally double. The proof is double the alcohol by volume. So a liquor at 80 proof is 40 percent alcohol by volume. A 100 proof is 50 percent alcohol by volume.
Proof originated from the British Navy during the ages of empire, where you would have gunpowder and rum in the same hold. If the rum barrel breaks, your gunpowder is going to be more or less useless, but over a given percentage of alcohol, 57 percent alcohol, 114 proof, you can still ignite the gunpowder even though it's wet. There's enough alcohol in there that it starts off a reaction, and, you know, your gunpowder still burns. Your pirates still go down.
So proof really only exists to this day in America. Some English and Scottish distilleries still use it, but generally people measure it in terms of alcohol by volume for the sake of international sales, and it's a lot less confusing. So a lot of stuff comes off at 80 proof. That's your standard. You know, that's your vodka proof, straightforward, easy-drinking. But there's a more interesting, you know, I mean more is more, so why stop at less? What we have is bonded whiskey. It comes off at about 100 proof, actually exactly 100 proof. And the proof system is still used for legal calcifications and classifications for bourbon. It has to come off at a given proof from the still and be barreled at a different proof.
But bonded whiskey is a fascinating concept. Essentially it's kind of a tax evasion scheme. The tax man still gets his. Don't worry, but what it does is it allows you to defer paying your taxes down the line. What you do is you put it into barrels. You put it in a U.S. government bonded warehouse for the years it takes to mature your product. And you don't pay taxes on it until it comes out of the warehouse rather than when it's made. So to be bonded, it's going to come off at 100 proof, sit in the barrel four years.
But you can also have, again more is more, higher proof whiskeys. This one, for instance, is 133.5 proof, which is 66.75 percent alcohol. It burns like the clap, and it's delicious. It's a very high proof liquor. You can smell it right away, you know, when you open the bottle. Even with this little whiskey in the bottle, I'm getting just the smell of alcohol all the way up. It comes out hot. Generally, you do want to dilute it a little bit or put it on ice. Put it on a rock.
Proof is a measurement of alcohol by volume, and the cask strength generally just means that it hasn't been watered down to proof. That's the strength at which it came out of the barrel. It's usually going to be hot. It's usually going to be aggressive, and if you have a taste for that sort of thing, it's a delicious thing.
And those are the basic facts about alcohol by volume or proof.