The cylinder head is one of the most important elements in internal combustion engines, being important for performing several main functions provided by the engine. It serves as a seat for the combustion chamber, it houses the valves, and leaves space for the passages where air-fuel goes in and out. Modern characteristics of cylinder heads are the use of higher quality and high heat resistance (alloy) metals in their construction to help resist warping; this is with a direct result of poorer metallurgy in older heads, such as iron or low grade aluminum. They also typically have multiple valves, other features including variable valve timing, double overhead camshafts and gasoline direct injection. Cylinder heads find applications in everything from automotive and motorcycle engines to industrial machinery, delivering optimized power output and efficiency at home in a variety of places.