Luggage: How much luggage can I bring and do I have to pay?
Let it be said right away - luggage rules are not easy to neither explain nor understand. Unfortunately not all carriers follow the same rules and there are a lot of exceptions to the general rules.
General rules are that you can bring one suitcase weighing 20 kg for free except for flights to/from North America (USA, Canada and Mexico) where you are allowed to bring one piece of luggage weighing maximum 23 kg. Some airlines have other rules though allowing either less or more luggage and a trend among airlines is to allow 23 kg no matter the destination (for instance SAS and Lufthansa). Other airlines have no free allowance at all and charge a fee for checked-in luggage - applies for a few european airlines and most airlines flying domestic in the USA.
If you are flying on more than one carrier on the same ticket (and not if you have separate tickets) most airlines follow the 'Most Significant Carrier' rule. This means in simple terms:
A) If you have a transit within your own area (for instance within Europe) it will be the luggage rules for the long sector which applies for all sectors (for instance from London to the USA).
B) If you travel within the same area (for instance Europe) it will be the luggage rules for each carrier which applies for the sector you fly.
This means you can have one set of luggage allowance to your destination and another allowance back from your destination. This means that it is always recommended to check your airline's specific luggage rules and prices at the relevant airline's website before you travel!
Be aware of maximum allowance, dimensions and weight for for checked-in luggage apply, which means your luggage cannot be of any size - for instance you cannot bring a flag pole even though it weighs less than the allowed kg's. If you want to bring more than the allowed pieces or weight you can pay for excess luggage.