Sweet vermouth is kind of like the acoustic guitar: it didn't require a modifier until a newer version came along. The products we know today by groupings such as sweet, Italian, and red vermouth originated in and around the city of Turin (aka Torino) sometime during the 1700s. Soon after, drier-style riffs on these products began to surface, prompting a need for finer classification.
Now we've reached the point, helped by renewed interest in old cocktails, where there's seemingly endless nerding out over nuance in store for the aspiring vermouth aficionado. But for now let's start at the beginning, with the stuff we're going to call sweet vermouth.
Sweet vs. Red Vermouth
The reason behind our choosing the modifier "sweet" over, say, "red" when discussing this product category has to do with cocktails — and the purpose of The 9-Bottle Bar, after all, is to help you make great cocktails at home! What's going to most affect the taste of a cocktail isn't a vermouth's color or origins so much as its degree of sweetness. A Manhattan, for instance, wouldn't be a Manhattan if the vermouth you reached for wasn't sweet.
What Is Vermouth, Actually?
But let's back up momentarily. As drinks writer Martin Doudoroff explains in his indispensable online guide Vermouth 101, vermouths belong to a category of aperitif beverage called fortified and aromatized wine. Vermouths include a grape wine base, typically derived from white varietals — even if the end product happens to be red in color. This base is infused with a blend of botanicals (i.e., aromatized) and made more alcoholic (i.e., fortified) with the addition of spirits. The name "vermouth" is actually a nod — via the German translation, oddly enough — to one of its original, primary botanicals: wormwood.
As Doudoroff points out, the process of fortification is what leads to the sweetness you taste in a sweet vermouth. As wine ferments, yeast consume grapes' sugars and — praise Mother Nature! — release alcohol. A sudden introduction of strong alcohol by the winemaker can halt fermentation, leading to higher levels of unconsumed, residual sugar in the wine than there would have been had the yeast been left to work their magic. Further depth of flavor and color comes from adding burnt sugar into the blend, hence the handsome red-brown tones of many sweet vermouths.
Europeans have long enjoyed sweet vermouths and other wine-based aperitifs on their own, to gear up their appetites for a meal. But in America, sweet vermouth has a much more prominent history as a cocktail ingredient. Its sweetness, aroma, and herbal undertones make it, among other things, a useful bridge between spirits and bitters.