Loire Valley, France
Like with many mature classification systems, the winemakers in Loire Valley have certain "rules" to follow to maintain the classification, leading to a few major grapes that dominate the Loire region, depending which region they are based in.
Lower Loire or Pays Nantais is all about citrusy, oyster pairing white wines. Close to the Atlantic Ocean, the maritime climate & rocky-sandy soils. Muscadet (Melon de Bourgogne) & Pinot Gris are dominant grape varietals here
Middle Loire is where you find the creme de la creme of Chenin Blanc, and where Cabernet Franc is revered. Anjou will give you more powerful wines, while Saumur has the perfect conditions for sparkling wines due to the limestone soils & caves
Central Loire is what most people think of when they think of Loire - Sancerre & Pouilly Fume. Sauvignon Blanc is without a doubt the msot important, while Pinot Noir & Gamay has had increasing importance for Rouge variants
Upper Loire is the most obscure region, with Gamay & Pinot Noir growing very well here. If you love a Beaujolais, look out for the Gamay from here, which can be slightly more powerful & have higher acidity