An aircraft cabin is the section of an aircraft in which passengers travel. At cruising altitudes of modern commercial aircraft, the surrounding atmosphere is too thin for passengers and crew to breathe without an oxygen mask, so cabins are pressurized at a higher pressure than ambient pressure at altitude. In commercial air travel, particularly in airliners, cabins may be divided into several parts. These can include travel class sections in medium and large aircraft, areas for flight attendants, the galley, and storage for in-flight service. Seats are mostly arranged in rows and alleys. The higher the travel class, the more space is provided. Cabins of the different travel classes are often divided by curtains, sometimes called class dividers, but not on all airlines. Passengers are not usually allowed to visit higher travel class cabins in commercial flights.