Can you explain what "first-growth" means?

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Dear Dr. Vinny,
Can you explain what "first-growth" means, and why these wines are considered better? Of all the literature I have read, I can't find a definition of this type of classification.
—D.B., Bronxville, N.Y.
Dear D.B.,
When people say "first-growth," they're talking about Bordeaux, or at least alluding to it. In the Bordeaux Classification of 1855 (yes, we're talking about a term coined more than 150 years ago), wine brokers ranked châteaus based on reputation and price. The classification ranked them within "growths," or crus, from first down to fifth. The original classification included four first-growths: Lafite-Rothschild, Latour, Margaux and Haut-Brion. In 1973, Mouton-Rothschild was elevated to first-growth (or premier cru) status, making a grand total of five first-growths.
It's no surprise that since 1855, many changes have occurred in names, owners, vineyards and quality. While the ranking still has a big influence on the perception of quality, these days it's not unusual to see a second-growth, or even a wine from a source ignored by the classification system, fetching as high a score (or a price) as a first-growth.
—Dr. Vinny