Wood has never been more important to whiskey than right now. Twenty years ago, you might find a single malt scotch that was labeled “sherry finished,” but details about the type of sherry, the size of the cask, and where it came from? That’s pretty new.

Whiskey drinkers want to know everything they can about how their drink was made and where its flavors came from, and distilleries are increasingly obliging. There’s a whole lot of vocabulary to learn—including some words you may not even realize are important to know. But once you understand what these terms mean, you’ll be clued into the flavors you can expect when you see them.

Let’s demystify the cask chatter and dial in on the wood words that count.

Cask Sizes and Shapes

In American whiskey, there’s one standard barrel size: 53 gallons (200 liters), and occasionally smaller sizes like 15 and 30 gallons. But overseas, especially in Scotland, cask types vary widely, thanks to the industry’s broad acceptance of using wood from wine, rum, American whiskey, and other spirits industries. Scotland’s whisky tradition also includes re-coopering old casks—breaking them apart, then refitting the staves into new configurations—so there are shapes and sizes you won’t find elsewhere.

Besides the 53-gallon American whiskey cask, which in Scotland is called an American standard barrel, there are other common types.

• Butts come from the sherry industry and measure 500 liters (132 gallons). These days, most butts are purpose-made for whiskey distillers and seasoned for a year or two with sherry just to add flavor. The wine that results, which is insanely oaky, usually gets turned into vinegar.
• Pipes are so-called because of the Portuguese word for cask: pipa. Naturally, they’ve usually held port previously, and can range in size, but are often 350 liters. Many pipes have an elongated slimness that underscores their name.
• Hogsheads run anywhere from 230 to 250 liters. Commonly referred to as “hoggies,” they’re made from disassembled bourbon barrel staves topped with new heads.

These aren’t the only cask sizes and shapes. There are also puncheons (of ambiguous capacity, though usually larger than a butt), quarter casks (usually, but not always, about a quarter of an American standard barrel, 50 liters), barriques (like those used in Bordeaux and Cognac), and, less commonly, drums, octaves, rundlets, firkins, kilderkins, and the deliciously macabre blood tubs. Some scotch distilleries, notably Balvenie and Glenfiddich, also employ tuns, which are neither a standard size nor actually maturation vessels, but are used as a sort of solera.

Barrel Uses

Bourbon, rye, and other straight American whiskies (except corn whiskey) must always be matured in new barrels. No such requirement exists anywhere else in the world, so everyone else ages pretty much all of their whisky in casks that have already been used once, twice, even three or four (or, in Canada, five or six) times.

• Used casks are usually described as “ex-” whatever: ex-bourbon, ex-oloroso, ex-Grand Cru Bordeaux. And there are specific ways to describe how many times a cask has been used for whisky maturation.
• First-fill means the cask is holding whisky for the first time. Thus, second-fill means it’s on its second round of maturing whisky. And so on. The term refill is intentionally vague, as the cask's previous number of uses may not be known.
• Occasionally non-American distillers opt for new casks, and many refer to them as virgin oak. For obvious reasons, this term is starting to fall out of fashion, with suggested alternatives including fresh casks, first-use casks, or simply new casks.

Cask Storage

The way casks are stored can have a profound effect on how the whiskey matures. If a warehouse has exposure to weather and temperature swings, the interaction between spirit and wood will be heightened, yielding barrel notes in the whiskey fairly quickly. If there’s little airflow or a steady, cool aging environment, interaction is much more restrained, and the whiskey will mature more slowly.

There are three main warehouse styles:

• Rackhouses, or rickhouses, are common in Kentucky, where the multi-story buildings hold tens of thousands of barrels. A rack is just a type of barrel shelf, with double slats fixed to vertical posts. Barrels are hoisted onto the slats and stored on their sides in long rows, with each set of racks stacking four or five high. There’s plenty of airflow between and around barrels, and the rackhouse’s design may incorporate features, like windows, to promote even more.
• Palletized warehouses turn the barrels on end, fitting four standard-sized American barrels on a pallet. The pallets are then stacked to the ceiling—literally, as high as they can go. There’s much less airflow in a palletized warehouse than a rackhouse, but the benefits of the design—namely, space savings and safety (everything is moved by forklift)—are enough that this has become the dominant type of maturation design across the global whiskey industry.
• Dunnage warehouses are found primarily in Scotland, though they hold only a tiny fraction of the scotch industry’s casks, and are probably kept around as much for their tourism appeal as anything else. These buildings are made of thick stone, with earth floors, which keeps the interior steadily cool and humid year-round. Casks are stacked on top of each other, just two or three high.

Barrel Burning

Although most American whiskey has been required to use new, charred oak barrels for decades, specifying the char level—and, increasingly, the toast level as well—has only recently become A Thing. Terms like “level four” and “alligator char” (because the burnt wood resembles a rough reptilian hide) are bandied about as if there’s an agreed-upon meaning for them. But—there isn’t.

Each individual cooperage that makes barrels has its own system of charring, usually based on how much time the barrel spends actively on fire. So level four at cooperage A may only be level three at cooperage B—and cooperage C may even offer a fifth level. (Shades of Nigel Tufnel.)

Get into toasting and it’s even more of a Wild West. Contrary to what some marketers would have you believe, bourbon barrels have been toasted as long as they’ve been charred—longer, in fact. It’s just that emphasizing the step, which is a prerequisite to charring, and amplifying it have become more popular of late. Cooperages are rolling out all kinds of customized toasting techniques, using everything from traditional fire to infrared ovens, to offer their distillery clients more varied options.

In short: even if you know what cooperage made the barrels a whiskey was aged in, understanding exactly what they did with the wood and what the results were is about as simple as higher math. Words like “heavy char” should indicate a whiskey with strong classic notes, like vanilla, and possibly some smokiness. A whiskey that plays up its toasted barrels likely features more prominent pastry flavors, like butterscotch, and sweetness. But given how varied the landscape is, what comes out of the bottle could still be quite a surprise.