The wines made in Sauternes, neighboring Barsac and three other adjacent communes, are the most renowned of all the world's late-harvest sweet wines, and are among the only ones made in commercially serious quantities. They are usually affected to some extent by botrytis, or ""noble rot,"" a beneficent fungus that increases glycerol and concentrates sugars and acids by dehydrating and shriveling the grapes. However, in years that don't offer the right climatic conditions (i.e., alternating humidity and dry heat), destructive gray rot, noble rot's evil twin, may ruin the grapes. Or, if there's no rot at all, an estate may merely be able to make a sweet wine that misses out on the nectar-like complexity and texture of classic Sauternes. Making great Sauternes is always a financially risky venture, as waiting for ideal botrytis always runs the risk of encountering rain or frost, which could ruin a season's crop.
Most of these sweet wines are based on the Semillon variety, which is prone to botrytis and yields rich, broad wines, often with a honeyed character. A percentage of Sauvignon Blanc is used to add acidity and freshness to most Sauternes, while some estates also like to blend in a bit of aromatic Muscadelle. The richness and unctuous quality of the best Sauternes belie an often high acid content, which, along with elevated alcohol levels, provides Sauternes with the structure to age remarkably well, in some instances for several decades. Not surprisingly, styles of Sauternes vary significantly by varietal make-up, the quality of the site, the seriousness of work in the vineyards, and approaches to vinification and aging.