If a wine is aged in 55 percent new French oak, what is the other 45 percent?

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Dear Dr. Vinny,
When a wine is “aged in 55 percent new French oak,” what does that mean? What is the other 45 percent?
—Mike, St. Louis, Mo.
Dear Mike,
There are usually a few words before or after that phrase to give “aged in 55 percent new French oak” some more context. But I’d reckon that all of this particular wine is aged in oak barrels, with 55 percent of those barrels being new, and the other 45 percent having already been used in previous vintages—those are usually referred to as used, or "neutral" oak barrels.
When a barrel is new, it will impart more and stronger flavors into a wine—those notes of vanilla, spice, cedar, tobacco and so forth. Older, neutral barrels can still add a rich texture to a wine while preserving the fruit flavors, and have a more subtle aromatic profile. Mixing the two is very common, but some wines are made entirely in new barrels, and other wines are made entirely in neutral oak. The size of the barrel also matters in this area of discussion—smaller barrels will impart more flavors because of the higher ratio of the barrel's surface area to the wine, while larger barrels and casks will have a more subtle influence.
There’s no right or wrong way to make (or prefer) wine, but if you start paying attention to these winemaking choices, it can help unlock some of the magic of why a wine tastes the way it does.
—Dr. Vinny