What’s the difference between Sémillon and Sauvignon Blanc?

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Dear Dr. Vinny,
What is the difference between Sémillon and Sauvignon Blanc?
—Christian P., Jakarta, Indonesia
Dear Christian,
Sémillon and Sauvignon Blanc are the names of two different white wine grapes (and of the wines made from them). They’re linked because they’re often blended together, as in Bordeaux blancs or wines inspired by them.
Sauvignon Blanc is definitely flashier than Sémillon. It has more intense fruit flavors, a more vivid acidity and a lighter body. Sémillon is heavier, with lower acidity and an appealing, almost oily or waxy quality to it. Blended together, Sémillon can give Sauvignon Blanc a terrific structure and weight, the two balancing each other out.
—Dr. Vinny