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On the commercial transportation, mostly with airlines, the baggage allowance is the amount of checked or carry-on luggage the airline will allow per passenger. On some airlines, this is the amount that is allowed free of charge. In other cases, this is the firm limit, and carrying additional weight for an extra payment is not an option.
The general allowance per passenger depends on the policies of the particular airline. On U.S. domestic flights, it was typical for an airline to allow passengers to check up to 2 pieces of lugagge that are up to 50 pounds (22.7 kg) each free of charge (total 100 pounds = 45.4 kg), and this can be exceeded for a fee. This changed during 2007 with most airlines now charging for both the first and second bag. However, within Europe, and often on flights between the United States and Europe, the limit is as low as 40 pounds (18 kilograms) total per passenger, and many airlines do not allow passengers to exceed this amount, even with payment of a fee. Many passengers complain about this limit, because after the typical 15-20 pounds (7–8 kg) of the suitcase itself, little room remains for any other items.
Checked luggage is usually measured by weight. All checked items are generally weighed by the airline, and if they exceed the limit, the passenger is informed by the airline. To avoid any fees, the passenger often must switch some of the items found in the suitcase to another suitcase, or else carry it on.
Carry-on luggage is judged primarily by size. Bags are measured by dimension or in total linear measurement (length + width + height). Passengers can thereby skip weight restrictions by carrying on the item. However, there are more restrictions on the types of belongings that can be carried on the plane.
On flights over the Atlantic Ocean (and on selected flights which vary by airline), passenger baggage is controlled by the so-called Piece Concept. Under the Piece Concept, passengers are permitted to check two bags with a per-bag weight of up to 50 pounds (23 kilograms) for Economy Class, and up to 70 pounds (32 kilograms) for Business or First Class. Certain airlines operating under the Piece Concept may add additional checked baggage allowance for their Elite Level fliers (see below sections). Weight restrictions are per bag, unlike the Weight Concept (described below). Most airlines have changed the policy and allow as of November 2009 only one bag to be checked in for free. The second bag will cost between $50–100. 
On flights not serving the United States or Canada (or other locations specified by the airline where the Piece Concept does not apply), passenger baggage is controlled by the so-called Weight Concept. Under the Weight Concept, each passenger is permitted to check a total bag weight in however many bags they have. Typically, Economy Class is permitted to check in up to 20 kilograms, Business Class is permitted to check in 30 kilograms, and First Class is permitted 40 kilograms. Some Airlines mainly based in the Middle East such as Emirates, Gulf Air, Qatar Airways, Etihad Airways, etc. offer 10 kg extra per class, so 30 kg Economy, 40 kg Business and 50 kg in First Class. Unlike the Piece Concept, in which weight restrictions are per bag, the Weight Concept allows passengers to combine their bag weight into fewer bags which would otherwise be too heavy to travel under the Piece Concept. For students some airline allow additional extra for each class. This would make the weight limit as 40 kg in Economy, 50 kg in Business and 60 kg in First Class; these are in addition to 7 kg cabin allowance. So in effect a student can carry a total of 47 kg in Economy Class, 57 kg in Business Class and 67 kg in First Class.
- http://www.iata.org/whatwedo/passenger/passenger_baggage/Pages/check_bag.aspx IATA Baggage Information
New IATA link for reference: http://www.iata.org/whatwedo/passenger/baggage/Pages/check-bag.aspx