A chimney is a structure used to ventilate hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere. Most chimneys are vertical, or almost vertical, to make sure that the gases flow easily, drawing air into the combustion in what is known as the stack, or chimney, effect. The space inside a chimney is called a "flue". Chimneys may be found in buildings, steam locomotives and ships. In the United States, the term smokestack is also used when talking about locomotive chimneys or ship chimneys, the word funnel can be used too.