St. Louis Barbecue Sauce is thinner and has more of a tangy flavor than its Kansas City cousin. Being at the crossroads St. Louis style barbecue has many influences, so there are a number of ways of making this style sauce.

Ingredients:

    * 2 cups ketchup
    * 1/2 cup water
    * 1/2 cup apple cider vinegar
    * 1/4 cup brown sugar
    * 3 tablespoon yellow mustard
    * 1 tablespoon onion powder
    * 1 tablespoon garlic powder
    * 1 teaspoon cayenne

Preparation:
Combine all ingredients in a saucepan over a low heat. Stirring occasionally and simmer for 20 minutes. Sauce should be thin, but not watery. Allow to cool. Store in an airtight container and refrigerate. Sauce is better if allow to sit for a day.

St. Louis-style ribs, cut from spare ribs, are a good example of such recipes. In preparation, the brisket bones are removed from the bottom of the rib rack. Slow cooking over low heat is the key to culinary success here, with a good, smokey grill. Saucing the ribs is a key to the distinctive flavor, with the liberal use of St. Louis-style barbecue sauce and returning the sauced and re-sauced ribs to the grill repeatedly for caramelizing.

snipped from: http://en.wikipedia.org/wiki/St._Louis-style_barbecue