How does a solera system work for making Champagne or fortified wines?

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Dear Dr. Vinny,
How does a solera system work for making Champagne or fortified wines?
—H.F., Malaysia
Dear H.F.,
A solera is a system of perpetual blending that is sometimes used in non-vintage bottlings of bubbly and fortified wine (as well as vinegar, whiskey and rum) as a way of keeping a consistent style.
To imagine how it works, picture a stack of barrels with the oldest barrels on the bottom and the youngest barrel on top. Wine is bottled from the barrels on the bottom, and the missing wine is then replaced with wine taken from the next youngest barrel, and then that barrel is topped off with wine from the barrel above it, and so on. New wine goes into the last barrel on the top every vintage.
When done right—most important, the barrels are never completely drained—the oldest barrels on the bottom will always contain a portion of the original vintage used in the solera—even if it’s only a trace amount. It also maintains a steady, complex style, continually blending a similar proportion of older wine with newer wine.
Solera production is pretty rare. It’s not always easy for a winery to hold back a lot of wine (they have more incentive to sell it than store it), and it’s easier to keep wine in one tank than dozens of individual barrels.
—Dr. Vinny