The major components of a gin and tonic are right in the name of the cocktail. Though tonic is clearly secondary, it cannot be ignored or waved in the vague direction of the spirit, as some bartenders do with vermouth when making a martini. Tonic’s flavors are assertive, easily discernible and crucial to the composition. Without it, a gin and tonic is simply a bad martini.

Even so, the tonic is easily taken for granted, which is fine — as long as you don’t pay attention to the gin, either, or the lime or the ice. For just as the quality and character of gins can vary widely, so, too, can tonics. The choice of a tonic produces very different cocktails, as I found in a recent blind tasting of six tonics.

The tasting took place, appropriately, at the Gin Palace, a cocktail bar in the East Village with 64 varieties of gin. I was joined by Rosie Schaap, the drinks columnist for The New York Times Magazine and author of the memoir “Drinking With Men.” Before each of us were six gin and tonics, all mixed by Frank Cisneros, the beverage director, all made with Beefeater gin, an excellent benchmark London dry gin.

Mr. Cisneros, naturally, takes tonic seriously. As with many high-end cocktail lounges, he makes his own (three, actually) and has Bittermens Commonwealth, which is a tonic liqueur, and Q, a good commercial tonic that was the third favorite in our tasting.