Just as with crude oil, there is a direct relationship between the molecular weight of the hydrocarbon fraction and its Btu content. In the case of natural gas, the lighter hydrocarbons of methane and ethane have the lowest heating values in Btu's per cubic foot. The heavy hydrocarbons of propane and butane have more Btu's per equal volume. Natural gas that contains significant amounts of C3, C4, C5 (pentane), and C6 (hexane) is a member of the wet gases. Natural gas that contains mostly methane and some ethane (C1 and some C2) is a member of dry gases. The terms wet and dry denote whether or not the gas contains hydrocarbons heavier than butane, which may be in liquid form.