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Checked baggage refers to items of luggage delivered to an airline or train for transportation in the hold of an aircraft or baggage car of a passenger train, which means it is inaccessible to the passenger during the flight/ride.
This baggage is limited by airlines with regard to size, weight, and number, usually dependent upon the fare paid, or class of ticket. Baggage exceeding the limits is regarded as excess baggage. In the US, checked baggage is no longer complimentary with most discount economy tickets, with the exception of Southwest Airlines and JetBlue Airways where the first bag is still complimentary as well as the second bag for Southwest. For other carriers, a passenger generally has to hold a full fare Economy ticket, travel in a premium cabin, or hold elite status on their airline to receive complimentary checked bags.
Passenger-baggage reconciliation 
According to the rules of most air transportation authorities, such as the U.S. Federal Aviation Administration and European Union's Joint Aviation Authorities, should passengers flying internationally with checked baggage fail to arrive at the departure gate before the flight is closed, that person's baggage must be retrieved from the aircraft hold before the flight is permitted to take off. In the United States, this does not apply to domestic flights since all bags are required to go through explosive detection machines (EDS) prior to loading. Making sure passengers board flights onto which they have checked baggage is called "passenger-baggage reconciliation" and is accomplished automatically through two commercially available systems. The security presumption of passenger-baggage reconciliation is that terrorists will not want to kill themselves, and will not board an aircraft if they have caused a bomb to be placed in its hold. This presumption does not hold true of suicide bombers.
Unaccompanied suitcases led to the downing of two flights, when a bomb inside the suitcase exploded:
Excess baggage 
Excess baggage is the amount of baggage that is in excess of the free allowance in size, number, or weight permitted for the air journey. At the airline's discretion, this may be carried at an extra charge, but no guarantee is made and it might have to be sent as freight instead. Some airlines impose excess baggage embargoes on certain routes, indicating that they will accept no (or very little) excess baggage.