An airline ticket is really a legal contract which specifies and restricts the services that passengers may expect and when they may expect them. On each ticket, the duties and liabilities of both passenger and airline are clearly stated - whether it is a scheduled or a charter flight - and each passenger must be in possession of a ticket for the journey to be undertaken. The Warsaw Convention limits the liability of most airlines in cases of injury or death involving passengers, and also of baggage loss or damage. This agreement is usually explained on the inside cover of the ticket or on a summary inserted in loose-leaf form.

The format of tickets issued by IATA-appointed travel agents in the UK and a number of other countries has been changed to conform with the requirements of the so-called Bank Settlement Plan (BSP). Instead of having to keep a stock of tickets for each airline with which they deal, agents now have one common stock of 'neutral' tickets, but a special plate is slotted into the ticket validator at the time of issue to indicate which airline is issuing the ticket. The whole BSP operation is essentially aimed at simplifying accounting procedures for both travel agents and airlines. Tickets issued direct by airlines still carry the normal identification.

Flight coupons contain a fare construction box which, on a multi-sector itinerary, indicates how the fare is to be apportioned among the different carriers. Cities are denoted by their three-letter codes, e.g. LHR is London Heathrow, ROM is Rome, CPH is Copenhagen, LAX is Los Angeles and so on. The fare construction may be shown in NUCs (Neutral Units of Conversion), a universal 'currency' in which fares are frequently expressed. The amount in NUCs is converted into the currency of the country of issue, which is shown in the fare box in the left-hand corner. The British pound sterling is shown as GBP so as to distinguish it from other sterling currencies. Where local taxes are to be paid these are also shown, and the final amount to be paid is shown in the total box.

At the bottom of the right-hand side is the 'Form of Payment' box. If you pay for the ticket by cash, this will either be left blank or the word 'cash' will be written in. If you pay by cheque, the word 'cheque' or abbreviation 'chq' will be used. If you pay with a credit card, the letters 'CC' will be entered, followed by the name of the issuing company, the card number and its expiry date. If you have an account with the travel agent the clerk will write 'Non ref', which means that no refund can be obtained except through the issuing office.

In the 'Baggage' section of the ticket, only the 'Allow' column is completed by the agent. This shows your free baggage allowance. The number of pieces, checked and unchecked weights are completed when the passenger checks in. 'PC' indicates that the piece concept is in operation, as it is on flights to and from North America. There are validity boxes immediately above the cities on your itinerary. These 'not valid before' and 'not valid after' entries relate to promotional fares with minimum/maximum stay requirements, and the relevant dates will be shown here. If you have a full-fare ticket where there is no minimum-stay requirement and the maximum is one year, these boxes are frequently left blank.

Immediately to the right of the itinerary is a column headed 'Fare/Class basis'. The letters most commonly inserted here are 'F' for First Class, 'C' for Business Class or 'Y' for Economy Class. The 'Y' may be followed by other letters to describe the fare, especially if it is a promotional type. For example, 'YH' would mean a high season fare, 'YZ' a youth fare, 'YLAP' a low season Apex, YE' an Excursion, etc.

Under the 'Carrier' box is the space for the carrier code, e.g. LH for Lufthansa or BA for British Airways. However, the airline industry has now run out of combinations of two-letter codes, so numbers have been added in conjunction with letters (e.g. Y2 is SA Alliance Air) and three-letter codes may be introduced in the future. Next follows the flight number and class of travel on that particular flight. Most international flight numbers consist of three figures, but for UK domestic flights four figures are frequently used. The date is written as, for example, 04 JUN for 4 June, while the time is shown according to the 24-hour clock, e.g. 14.30 hrs and not 2.30 pm (though the 12-hour clock is still used for domestic travel within the USA).

The 'Status' box will contain the letters 'OK' if the flight has been confirmed, 'RQ' if requested but not yet confirmed, and 'WL' if it has been wait- listed. If you have not decided when you want to travel, the word 'OPEN' will be written across the flight number, date, time and status boxes. Infants, who travel for a ten per cent fare on international journeys, are not entitled to a seat or baggage allowance, so the reservations entry will be marked 'No seat' and the allowance marked 'nil'.

Your ticket is valid for travel only when date-stamped with a travel agency or airline validator, complete with the clerk's signature or initials.