Clam chowder is any of several chowder soups containing clams and broth. In addition to clams, common ingredients include diced potatoes, onions, and celery. Other vegetables are not typically used, but small carrot strips or a garnish of parsley might occasionally be added primarily for color. A garnish of bay leaves adds both color and flavor. It is believed that clams were used in chowder because of the relative ease of harvesting them. Clam chowder is usually served with saltine crackers or small, hexagonal oyster crackers. (source)