If you are packing luggage with toileteries, clothes, and other items that will be checked-in instead of toted as part of your carry-on, you can refer to basic information found on your ticket. There are two very common terms on tickets, 2PC and 20K (rarely 10K).
2PC means that you are allowed to check in two pieces of luggage, normally 70lbs or 32kg per piece, and increasingly more common is 50lbs per piece. 20K (or another number like 10K for example) is a weight restriction, which means that the total weight of your checked in luggage can not exceed 20kg, or 44lbs.
On a paper ticket, you can find this information by looking for a column that is titled ALLOW., ALLOWANCE, or something similar. On the paper ticket, this column is usually located in the last column on the right hand side of the ticket.
For ticketless travel, this will appear on one of the documents that the airline emails/faxes/mails to you, and although each airline is different, this information is usually on the page that contains detailed fare information. If not, look through the contract of carriage, or conditions of contract document.
So you are ready to check in - now what? Baggage tags are placed on your checked in luggage in order to tell the luggage handlers (and the airline) where your luggage is supposed to go. Three letter airport codes were introduced to make it easier to send luggage to its rightful destination. This link to airport codes will allow you to look up the airport codes that will be part of your air travel experience, so you can take a quick look at the tag to make sure it is being sent to the correct destination.
If you are making connections, or have more than one ticket (for example you have one ticket for Chicago O'Hare, ORD, to London Heathrow, LHR on United Airlines and one for London Heathrow, LHR to Geneva, Switzerland, GVA on British Airways), do mention it at check in so that the airport agent can check your luggage through to your final destination. If you are connecting internationally, ask if you need to pick up your baggage on route - some countries allow baggage to go through, where others require you to pick it up, bring it through customs, and drop it off somewhere else, even when it has been tagged through to your destination.
You will be asked security questions at check in, which may include the specific contents. One item that is often confiscated is cooking oil (because of its highly flammable nature), so if you do decide to buy gallons of olive oil in Italy, don't be surprised if it is taken away from you at check in.
NEXT - Carry-on luggage
