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An appellation is a wine growing region. In the United States they are called American Viticultural Areas. These are the official grape growing regions that have been designated by the Bureau of Alcohol, Tobacco, and Firearms (ATF).
When a label lists an AVA, 85% of the grapes used for that wine must come from the AVA. AVAs are geographic areas that share the same soil, climate, elevation and other properties and give the wines particular characteristics. However, being from a particular AVA isn’t a guarantee of quality.
AVAs can be subdivided into smaller sections. For example, Stags Leap is a subdivision of Napa Valley. As you learn about different AVAs, you may discover that certain grapes grow well in specific AVAs. For example, Willamette Valley in Oregon is know for Pinot Noir and the Finger Lakes AVA is known for its great Riesling.
The largest AVA is the Ohio River Valley, which spans Ohio, Indiana, Kentucky and West Virginia with 16 million acres. More well known is the sprawling Columbia Valley AVA in Washington and bits of Oregon which has 11 million acres.
Some of the best known AVAs are in California including Napa Valley, Russian River Valley, Cry Creek Valley, Rutherford, Sonoma Valley, Mount Veeder and Howell Mountain.