How many different types of grapes are used to make wine?

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Dear Dr. Vinny,
Where would I find a list of the different varieties of grapes used to make specific wines like Merlot, Shiraz, Cabernet, etc.?
—Neil T., Nottingham, England
Dear Neil,
There are thousands of grape types out there, though I’ve heard that the number used to make wine commercially is probably closer to 1,300. If you’re trying to track them, it gets pretty complicated quickly—some grapes are clones of others, and some grapes have different names in different places. (“Syrah” and “Shiraz” may be the best-known example, but there are plenty of others.)
Depending on how much research you want to do, there are some great books on the subject. Or, you can check out Wine Spectator’s handy guide to the most prominent grapes and their characteristics.
—Dr. Vinny