In the seventeenth century, English merchants began what would later become a substantial trade with Portugal. As they searched for products to export to the British Isles, they sailed up the Douro River valley on the backs of boats known as barcos rabelos. What they found was a sweet nectar that delighted their tongues and captured their imaginations—a rich, fortified wine they called "port," after the city of Oporto at the mouth of the Douro.

The cool Atlantic winds bring much rain to the lower Douro valley, but further inland the Serra de Marão mountains begin to shield the valley from these damp breezes. As a result, the climate in the Upper Douro valley is hot and dry, and it is here that one finds the best vineyards. It is a commonplace in winegrowing that extreme conditions "stress" the vines, thereby coaxing them to produce better grapes, rather than more leaves. The conditions along the Upper Douro are excellent, affording just enough moisture to let the vines produce grapes of surpassing quality. The vineyards themselves are built on the granite hillsides, where farmers have carved out terraces from the softer schist over the course of centuries. The estates, or "quintas," are packed tightly together, making the most out of limited space.

As noted above, ports are fortified wines--this is a reference to the distinctive process used to make them. The grape juice is allowed to partially ferment, but this fermentation is cut short by the addition of either pure alcohol or brandy. The result is a sweet wine (the result of sugar left over from the halted fermentation) with a relatively high alcohol content (from the added spirits), usually in the range of 20%. Unlike other wines, ports are not always marked with a vintage—ports with a labelled year represent grapes from superior harvests and are only declared a few times each decade. Recent quality vintages include 1997 (whose ports are getting rave reviews), 1994, 1992, 1991, and 1985.

Ports can be made of either red or white grapes, though reds are much more common. Dozens of red grape varieties can be use to make ports, but the primary types are:

The most common grapes used to make white port include:

There is a wide variety of ports that are made from red grapes. These fall into two primary categories described by their general color: tawnies and rubies. Tawny ports are often aged in wood for several years before being bottled, and it is this time, spent in close contact with oak, along with carefully orchestrated exposure to the air, that lends tawnies their light-brown color and nutty flavor. Tawnies are often sold at several quality levels :

  • Bottles simply labelled tawny are often a blend of white and ruby ports and are likely to be seriously crappy.
  • Age-designated tawnies are sold as "10-year," "20-year," and so on. This means that the average age of the port in the mix is what's on the label.
  • Colheitas are tawnies whose wine all comes from a single year and which have been barrel aged for a minimum of seven years. The top-of-the-line tawny.

Ruby ports are distinguished by their color. The primary types include:

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