Cream of tartar is both a leavening agent (like baking soda and baking powder), and also adds a slight tangy flavor to baked goods. You can either replace cream of tartar with baking powder at a 1:1.5 ratio (1 teaspoon cream of tartar : 1 1/2 teaspoons baking powder), or you can replace cream of tartar with the combination of baking soda and either lemon juice or vinegar (as with this recipe). Because the cream of tartar in a classic snickerdoodle recipe provides a slightly tangy flavor, we chose the combination of baking soda and lemon juice as a substitute for cream of tartar in this recipe.