"Help," our caller said "I need a recipe for making buttermilk. Not the one that uses vinegar or lemon juice; the one that uses cream of tartar." While the staff of the Newton Falls Public Library was more familiar with the latter, they had heard of the former.
We were successful with an online search using the terms "buttermilk cream of tartar substitute." The website JoyTheBaker.com had three options for creating buttermilk. They included the familiar one using lemon juice, the one requested using cream of tartar, and a recipe with yogurt. Of the three, the recipe our patron needed said to "Mix 1 cup of milk with 1 3/4 teaspoon cream of tartar. To ensure that the mixture doesn’t get lumpy, mix the cream of tartar with 2 Tablespoons of milk. Once mixed, add the rest of the cup of milk."
If you do not have Internet access or the library is closed, when you run short of certain recipe ingredients while preparing a meal, emergency substitutions can be found in many home cookbooks. Substitutions and measurement equivalents can often be located on the inside covers of books like "New Cook Book" by Better Homes and Gardens.