Which grapes are allowed to be blended in Châteauneuf-du-Pape?

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Dear Dr. Vinny,
I could have sworn Viognier was blended with Syrah in Châteauneuf-du-Pape, but a friend of mine told me I was wrong. Aren’t red and white grapes blended together there?
—Alex, Decatur, Ga.
Dear Alex,
You’re right that in the Southern Rhône region of Châteauneuf-du-Pape, red and white wine grapes can be blended together—with delicious results. There are either 13 or 18 total grapes permitted, depending on how you count them, for both red and white wines. Originally, the vines were planted mixed together on the same plot, which explains why the region allows so many different grape varieties.
There are two numbers because some grapes have multiple color mutations. For example, Grenache has white, pink/gray and black grape versions, and all are allowed in Châteauneuf-du-Pape. Likewise, Clairette has white and pink versions, and Picpoul has black, gray and white mutations. But most people refer just to the primary names, so—drum roll, please!—let’s count off the 13 grapes.
By far, the most widely planted grape in the region is Grenache. You’ve probably heard of some of the other grape varieties, including Syrah, Mourvèdre and Cinsault. There’s also Roussanne, Clairette, Bourboulenc, Vaccarèse, Counoise, Muscardin, Terret Noir, Picpoul and Picardan. Note that while the appellation rules allow all 13 of these grapes, they’re not all required, and there’s no minimum or maximum percentage of any one grape variety.
I used to have a colleague who joked that half of these sound like Superman villains—but there’s nothing to be afraid of! They are typically blended in much smaller quantities with the three primary grapes of the Southern Rhône: Grenache, Syrah and Mourvèdre. That said, some producers are doing great things bottling these grapes, especially Clairette, on their own.
As far as Syrah and Viognier, they can absolutely be blended together into one wine, but those bottlings are from the Northern Rhône, especially Côte-Rôtie. That style of blend has also become popular in Australia, California and elsewhere.
—Dr. Vinny