What is a common mixer that is often used in highball cocktails?

Club soda is a common mixer frequently used in highball cocktails due to its effervescence and neutral flavor. Highball cocktails are typically characterized by a base spirit combined with a larger proportion of a mixer, and the carbonated nature of club soda complements the alcohol without overshadowing its flavor. This makes it an ideal choice for maintaining the crispness and lightness that many highballs strive for.

In contrast, options like orange juice and cranberry juice, while popular mixers in their own right, are more commonly associated with specific cocktails that call for fruit-based flavors rather than the simplicity and balance offered by club soda. Grenadine, on the other hand, is a syrup used for color and sweetness rather than as a primary mixer in highball drinks. Thus, club soda stands out as the quintessential mixer for highball cocktails.