From the Atlantic:

Cheaper wines are often made from blends of grapes, and there is a good reason for this: a skilled blender can coax good wine from mediocre grapes. Most prestigious and expensive vintages of wine are varietals, which means they're made from just one variety of grape.

Those who can afford it will often keep track of which years were good for which types of grape from which areas, and will pay good money for excellent wines. But if they aren't careful, and lucky, they'll also end up paying silly prices for mediocre wines. In contests, blended wines don't usually compete against varietals, because that would be unfair.

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