Airlines vary in size, from small domestic airlines to full-service international airlines with double decker airplanes. Airline services can be categorized as being intercontinental, domestic, regional, or international, and may be operated as scheduled services or charters. The largest airline currently[when?] is American Airlines Group.
Domestic flights are generally cheaper and shorter than most international flights. Some international flights may be cheaper than domestic ones due to the short distance between the pair of cities in different countries, and also because domestic flights might, in smaller countries, mainly be used by high paying business travellers, while leisure travellers use road or rail domestically.
Some smaller countries, like Singapore have no scheduled domestic flights.