Col fondo, pét-nat, traditional sparkling wine: What's the difference?

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Dear Dr. Vinny,
What's the difference between col fondo Proseccos, pét-nats and méthode traditionnelle sparkling wines?
—Damien S., Australia
Dear Damien,
Col fondo, pétillant naturel and méthode traditionelle are all methods for making sparkling wine, but they're each a little different.
Méthode traditionnelle, or traditional method, is a way to make Champagne and sparkling wines where a secondary fermentation takes place inside the bottle, and that's where the bubbles come from. It’s the most labor-intensive method for making sparkling wine: a fully fermented base wine is created, and the secondary fermentation is triggered by the addition of a liqueur de tirage, a solution of wine, sugar and yeast. That secondary fermentation also yields additional spent yeast cells, or lees, which are removed via two processes called riddling and disgorgement, by which the lees are collected in the neck of the bottle, frozen and then ejected, to be replaced by another solution of wine and sugar (no yeast this time) called the dosage. That's also what can give some sparkling wines their sweetness. You may have also seen the trend in racy, bone-dry Champagnes and other sparkling wines marketed as "zero dosage," "brut nature" or "sauvage."
Pétillant naturel, or pét-nat, is a type of sparkling wine that also undergoes a fermentation in the bottle, but it's a simpler method than that used for traditional Champagnes. Pét-nats are simply bottled before the primary fermentation is complete, so they aren't riddled or disgorged, either—and they're frequently bottled under crown caps (like a beer bottle) rather than corks.
Col fondo is a term reserved specifically for Proseccos (while the terms "pét-nat" and "méthode traditionnelle" are applied to sparkling wines from all over the world). A col fondo Prosecco starts with a base wine, then there is a secondary fermentation, but the distinguishing feature is that the wine is not disgorged—that is, the lees are left inside the bottle. The resulting wine can be a bit cloudy, and more sour than a typical Prosecco. Col fondos are also frequently—but not always—bottled under crown caps.
—Dr. Vinny