Clam chowder is any of several chowder soups in American cuisine containing clams. In addition to clams, common ingredients include diced potatoes, salt pork, and onions. Other vegetables are not typically used. 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. The dish originated in the Northeastern United States, but is now commonly served in restaurants throughout the country. Many regional variations exist, but the three most prevalent are New England or "white" clam chowder, which includes milk or cream, Manhattan or "red" clam chowder, which includes tomatoes, and Rhode Island or "clear" clam chowder, which omits both.