Clam chowder is any of several chowders containing clams and broth. Along with the clams, diced potato is common, as are onions, which are occasionally sauteed in the drippings from salt pork. Celery is frequently used. Other vegetables are uncommon, but small carrot strips might occasionally be added, primarily for color. A garnish of parsley serves the same purpose. Bay leaves are also sometimes used as a garnish and flavoring. It is believed that clams were added to chowder because of their relative ease to collect.
New England clam chowder is a milk or cream based chowder, that is traditionally made with potatoes, onion, bacon or salt pork, flour & clams. Adding tomatoes to clam chowder was shunned, to the point that a 1939 bill making tomatoes in clam chowder illegal was introduced in the Maine legislature. It is occasionally referred to as Boston Clam Chowder in the Midwest.
Manhattan clam chowder has clear broth, plus tomato for red color and flavor. In the 1890s, this chowder was called "New York clam chowder" and "Fulton Fish Market clam chowder." The name "Manhattan clam chowder" became attached in the early 1900s from the people in Maine. Restaurants typically serve New England or Manhattan chowder, but not both. Clam chowder, in its cream-based New England version, has been around since the mid-18th century, and no mention of any Manhattan chowder has been found that predates the 1930s. Many restaurants in northern Rhode Island sell both red and white chowders, while the southern coast favors clear and white chowders. Often they are served alongside clam cakes.
According to Good Eats, the addition of tomatoes in place of milk was initially the work of Portuguese immigrants in Rhode Island, as tomato-based stews were already a traditional part of Portuguese cuisine. Scornful New Englanders called this modified version "Manhattan-style" clam chowder.
Rhode Island clam chowder has clear broth. Though less popular than the other two, clear chowders are still served, especially at long-established New England restaurants and hotels, such as those on Block Island, and on the south coast of the state, where tourists favor white chowders while natives prefer the clear.
Some restaurants also serve their own unique clam chowders that do not fall into any of these three types. Clam chowder is usually served with saltine crackers or small, hexagonal oyster crackers. Throughout the United States, creamy New England-style clam chowder is sometimes served in sourdough bread bowls, especially in San Francisco where sourdough is popular with tourists and has been considered a signature dish since 1849.