How do winemakers age a wine in both French and American oak barrels?

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Dear Dr. Vinny,
What’s the meaning of combining, for example, French and American barrels to produce wine? Do they use two different barrels, one of each? Or is it a combination of the two in a unique barrel, hence reducing the cost?
—Diego, London
Dear Diego,
Different types of barrels impart different types of aromas and flavors into a wine. French and American oak are among some of the most popular types of wood used and, broadly speaking, French barrels are known for imparting more subtle spicy notes, while American barrels are stronger, giving more vanilla and cream soda accents.
If a winemaker wants to have more than one type of barrel’s influence in a wine, they can use multiple types of barrels—say, split up production by putting a percentage of the wine in one type, and the remainder in a different type. There are also hybrid barrels, made up of more than one type of wood—usually the top is one kind and the sides are different. But hybrid barrels aren’t cheaper, and in fact can cost more than some uniform barrels.
—Dr. Vinny