Fireplaces are architectural elements or functional structures that are designed to contain a fire for heating or decorative purposes. Traditional wood-burning fireplaces are made of masonry and built into the wall of a house, where they also provide an internal opening to a chimney. Free-standing fireplaces are made of metal and ceramic, and use wood, gas, oil, or electricity as their fuel source. Most fireplaces include a firebox, foundation, fireplace mantel, hearth, ash dump or cleanout door, grate or iron bars, lintel, damper, smoke chamber, throat, flue, chimney case, crown, spark arrestor, and cap or shroud. In the United States, fireplaces should comply with state, local and (in some case) federal standards for emissions. Selecting fireplaces requires an analysis of venting options. Direct-vent fireplaces have a piping system which consists of a smaller pipe inside of a larger pipe. The combustion air is drawn-in from the outdoors through the larger, outer pipe. The smaller, inner pipe vents the products of combustion horizontally through an outside wall, or vertically through the roof and out a chimney. Vent-free fireplaces are gas-burning appliances that do not require a chimney, flue, or vent. Some products do not even require electricity. Instead, the natural gas or propane flows through a permanent line that is connected to a burner within the heating appliance. B-vent or natural vent fireplaces are also available. They use the air inside of the room for combustion. B-vent fireplaces cost less than direct-vent models, and are available in traditional designs. Wood fireplaces, wood stoves, and wood fireplace inserts use wood as a fuel source. Products are characterized as catalytic or non-catalytic. Catalytic wood fireplaces and catalytic wood stoves have ceramic, honeycombed chambers that are coated with a metal such as platinum or palladium. Catalytic combustors then burn away gases and airborne particulates. Catalytic wood-burning appliances also allow wood to burn at lower temperatures for longer periods of time. With non-catalytic wood fireplaces, combustion occurs in the firebox, the part of the appliance where the fuel is located. Suppliers of wood-burning fireplaces may also sell pellet stoves, heating products that burn pellets made of compacted sawdust. Some fireplaces use gas, oil, or electricity instead of wood as the fuel source. Gas fireplaces may use natural gas or propane. They feature gas logs, which consist of a gas burner, grate, burner system, and mold which resembles firewood. Typically, this mold is made of refractory cement, ceramic clay, or ceramic fibers. Features for gas fireplaces include electronic ignition systems, standing pilot lights, and remote control. Electric fireplaces, electric stoves, oil fireplaces, and oil stoves are also available.