Sometimes, there are occasions when it amounts to sacrilege to mix one grape with another and so lose out on the essence of a particular variety. On the other hand, it can be a virtue.
It's the reason why buying a wine made from a blend of grapes may be greater than the sum of the two, or more, parts.
Most famously, Bordeaux red is a blend of Cabernet Sauvignon and Merlot. The wines of southern France are popular because, in the main, Syrah and Grenache are easy soulmates. Châteauneuf-du-Pape can utilise 13 grapes if it so wishes.
While Rioja red only allows for Tempranillo, a lighter regulatory touch elsewhere in Spain sees it with a variety of other grapes, often to better effect.