What's the difference between Port and Sherry?

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Dear Dr. Vinny,
What's the difference between Port and Sherry?
—Mike, Brooklyn, N.Y.
Dear Mike,
Port and Sherry are both fortified wines—that means that distilled spirits like brandy are added to the wine while it’s being made. That fortification increases the alcohol content and also stops the fermentation process, which leaves some residual sweetness as well.
But Sherry and Port both come from different places and are made in different ways. Sherry is a fortified wine from the Jerez region of Spain, where the primary grape is Palomino; while the wine is fermenting, a layer of yeast called flor is allowed to form on top of the wine, protecting it from spoilage and oxidation (although most Sherries are made in an oxidative style). Most Sherry is then aged in a solera system, in which vintages are blended together to ensure the consistency of the final product. For more, read our ABCs of Sherry.
Port, on the other hand, comes from Portugal's Douro Valley, where the primary grapes are Touriga Nacional, Touriga Francesa and Tinta Roriz (Tempranillo), but more than 80 varieties are permitted. In some years, the vintners of the Douro Valley "declare" the vintage to be of particularly excellent quality and will make Vintage Port, but most Port is also blended across vintages for consistency. There are many different styles of Port, which you can read about in our Port Primer.
—Dr. Vinny