What ingredient is primarily used to create a "sour" drink family?

The "sour" drink family is fundamentally characterized by the use of citrus juice, which provides that essential tartness and brightness that defines the flavor profile of these cocktails. Citrus juice, such as lemon or lime juice, acts as the acidic component that balances sweetness and often complements other spirits used in the drink. This balance is a critical aspect of creating a well-rounded sour cocktail, such as a Whiskey Sour or a Daiquiri, where the citrus juice plays a pivotal role in achieving the desired taste.

The use of simple syrup, although it can add sweetness to a sour cocktail, does not define the sour category itself. Likewise, while gin can be used in various sour drinks, it is not the defining ingredient. Bitters can sometimes be added to enhance flavor, but again, they do not serve as the primary ingredient responsible for classifying a drink within the sour family. Thus, citrus juice stands out as the essential element that signifies and creates sour cocktails.