When it comes to understanding oak’s influence on Scotch, forget everything you learned about its effect on wine.

Here's what wine drinkers believe about wood: European oak is subtle and toasty; American oak is strong and laced with vanilla.

No wonder Bourbon has such strong oak influence, while Cognac's oak notes are fewer.  That makes sense, right?

Now here's the inverted world of Scotch aging: If you want to taste the essence of a single malt, look for a version that was aged in used Bourbon barrels made from American oak.

And if you want a product that's tarted up to mask the real flavor, buy a Scotch that was aged in a used wine barrel made from European oak.

The language used on some Scotch bottles is a tip-off to this.  The Macallan's main products are 12-year-old and 18-year-old whiskies "exclusively matured in selected Sherry oak casks from Jerez, Spain."

In between those two products is a less common bottling called 15-year-old "Fine Oak," which is "carefully matured in a unique combination of Bourbon and Sherry oak casks." In fact, it's all American oak, even the barrels that were used to mature Sherry.

"American oak, in Scotch, gives a lighter, softer note," says Macallan whisky maker Bob Dalgarno.

The Macallan likes the effect so much that it buys and ships American oak to Jerez for Sherry makers to season.  And they're not alone; worried by a limit to the amount of harvestable forest in Europe, other Scotch makers have taken to supplying Sherry makers with oak as well.

It's nice to think of this as a total product cycle, with both Sherry and Scotch coming from the same barrel, but it's really all about the Scotch.  Top-quality Sherries don't come from new barrels; much of the Sherry used to season the barrels for Scotch is distilled into bulk alcohol by the EU, or simply poured down the drain.

American oak barrels previously used for Bourbon are far more prized than the ones from Jerez.  Diageo is the largest producer of single malts in Scotland, owning 28 separate distilleries.  Diageo maintains its own cooperage and employs about 90 of Scotland's more than 200 licensed coopers -- and they spend much of their time hammering apart old Bourbon barrels and refashioning them into different-sized Scotch casks.

The barrels are ugly when they arrive: blotched with white, brown and black stains, irregular and pot-bellied.  They're not at all like the beautiful new barrels you see in modern wineries.  It makes me think of organic agriculture, where ugly is the new beautiful -- these barrels are like an unkempt, unsprayed vineyard, better able to transmit the pure flavors of the "new-make" spirit of Scotch.

The inside of the barrels is cleaned before use with a steel rope, and it's re-toasted with a gas flame.  But the outsides are left blotchy, and the best Scotch comes from these ugly barrels.

I like elegance in Scotch, and for me the 15-year-old Fine Oak was the best version of The Macallan I tasted during a recent tasting trip in Scotland.  It was fresher and fruitier than the 12-year-old, with notes of rhubarb, apple pie filling, raisins and walnut pie. 

There's no better place to test the theory that American oak is the mildest than The Macallan's complete opposite, Glenmorangie, which sells small-lot versions of its spirit aged in used Sherry, Port and Sauternes barrels. 

It's amusing to visit both The Macallan and Glenmorangie.  They're owned by rival corporations (The Edrington Group and LVMH respectively), and each is so proud of the extreme shape of their still that they have a model of the other's in their distillery.  The Macallan claims to have the smallest still in Speyside; Glenmorangie has the tallest still in all of Scotland. 

The short Macallan still allows thick, heavy spirit to make it into the neck and ultimately the final product, which gives the whisky an oily, aromatic, somewhat rugged character; it takes the "Fine Oak" 15 years to soften it.  In contrast, only the lightest spirit makes it all the way up through the neck of Glenmorangie's stills, so the "new-make" whisky ends up light and lively.

Glenmorangie's new-make spirit was my favorite of several that I tasted; it's like a nectarine-and-pear eau de vie.  Master blender Rachel Barrie says the character comes from the mineral-rich spring water used to make it.  The point is, if the new-make spirit is that good, the barrels shouldn't cover it up.

But they do, on purpose.  The versions finished in used Sherry and Port barrels taste like Sherry and Port; meh.

I liked the Sauternes-finished Glenmorangie a lot -- it's sumptuous, like Sauternes, with notes of honey and apricot and honeycrisp apple.  Down underneath all that rich sweet fruit is a bit of the brush pine and salt air of Scotland.  (FYI; this is one of those rare Scotches that is better without adding water.)

But it wasn't as delicious, or as genuine, as the plain 10-year-old Glenmorangie, light and refreshing with notes of pear, green apple and apricot, and a long, elegant finish.

How does Glenmorangie maintain that initial liveliness, rounding it over 10 years in oak without overcoming it?

The answer is used American wood.  Glenmorangie owns forests in the Ozarks.  It harvests slow-growth oak and gives it for free to Jack Daniel's, which ages its Jack Daniel's Single Barrel Select whiskey in it for four years.  After that the barrels are shipped to Scotland to hold Glenmorangie.

I could give more examples, but it's more useful to give a general rule: If you want to taste the essence of a Scotch, look for one aged in used Bourbon (or Tennessee whiskey) barrels.  They're like a Ewan McGregor movie made in Hollywood: another great Scottish-American collaboration.