An A to Z for the air traveller: How to get from Alpha to Bravo without being a Charlie: this is Simon Calder speaking the airlines' language

A is for Apex, short for Advance Purchase Excursion, which is the standard cheap ticket on scheduled flights. You must book a couple of weeks in advance, stay away at least one Saturday night and pay heavily to change reservations.

Within Europe, Apex tickets are usually cheaper than normal one-way fares. But you can usually save more by not buying direct from the airlines. Carriers unload seats through discount travel agencies, or 'bucket shops'.

Any half-decent agent can obtain a less restrictive ticket at a lower fare than Apex. This includes the High Street multiples such as Thomas Cook, Pickfords and Lunn Poly, but discount specialists such as Major Travel (071- 485 7017) can often do better.

The best source of fares is Danns Digest, which is to air travel what Glass's Guide is to the motor trade. It lists the leading consolidators for destinations. Call 0435 867107 to subscribe.

B is for business class. The fare for business class can be 10 times higher than Apex. It buys more than a larger seat, bigger peanuts and obsequious cabin crew; it can get you an extra journey. A BA Club World return flight from London to New York costs pounds 2,122, but you can add a side-trip in Continental's business class from New York to Mexico City for not one peso more. The technical term for this phenomenon is the Maximum Permitted Mileage rule.

C is for couriers. Express delivery companies use casual couriers to maximise speed. The best way to send urgent documents is as accompanied baggage on a scheduled flight. The consignment can be checked in at the last moment and whisked through Customs with a minimum of fuss at the destination. This requires a fare-paying passenger, prepared to put up with a modicum of inconvenience in return for a low fare.

The companies to call are Polo Express (081-759 5383) and CTS (071-351 0300). They act as recruitment specialists for the courier companies. Washington DC this week is pounds 220 return, against pounds 350 for the lowest discount fare. Bookings are taken up to three months in advance, and early reservation is essential.

Couriers must check in at least two hours in advance, and look reasonably smart - no jeans or trainers. If your idea of a long- haul flight is a seven-hour binge, you're in the wrong job: couriers promise they 'will not consume excess alcohol in flight'.

If you use courier flights frequently and prove a sober, reliable sort, the agents may drop travel pearls in your lap. Susan Griffith, author of Work Your Way Around the World, is an experienced courier. 'Get on the inside track with a courier company,' she advises, 'and they start offering all sorts of good deals at quite short notice.'

D is for domestic flights. You have to be flexible to get good deals on flights within the UK. The most popular ones are early in the morning and on Friday and Sunday evenings. At other times, airlines discount heavily to fill the hundreds of empty seats shuttling back and forth. Bucket shops never discount flights within the UK, so approach the airlines direct.

Standby tickets have disappeared, except on British Airways Super Shuttle and British Midland domestic routes. For a 25 per cent discount, standby passengers are made to feel like second- class citizens. You are segregated and given a garish yellow boarding card. Once on board, you still expect to be tapped on the shoulder and asked to surrender your seat to a busy full- farepaying executive, but this rarely happens.

E is for executive clubs. Airlines reward passengers who spend large amounts of cash with them, and one of the most tangible benefits is the use of a lounge before a flight. The usual qualification is proof of extensive travel over a year. BA's Executive Club and KLM's Flying Dutchman Club have three tiers of membership depending on the amount of travel. The plebs are 'Blue' members, allowed nowhere near the free booze. The 'Silver' tier gets you a seat in the lounge, plus prority check-in. 'Gold' members can virtually select the crew.

F is for Frequent Flyer schemes, often linked to executive clubs. You collect points every time you fly, and trade these in for free flights. 'Freeway', Virgin Atlantic's scheme, recognises that after flying thousands of miles you might want something other than another flight, so it offers goodies such as polo lessons.

Frequent flyers often complain about the difficulty of claiming awards. Limited numbers of seats are allocated for free flights, and these seem to be at inconvenient times such as Tuesday afternoons in November. The danger is that you might not get round to claiming your prize before a carrier goes out of business. Anyone want 25,000 Pan Am Worldpass miles?

G is for 'green' air travel. No aircraft has yet been invented that does not harm the planet; all you can do is try to minimise the damage. The most obvious pollutant is noise, but improved technology means only Concorde and some ageing Russian jets bend the needle of the decibel meter.

Atmospheric pollution is a worse threat. A Boeing 737 flying between the UK and Greece consumes 10 tons of kerosene, i e, roughly 20 gallons per passenger is burnt and the emission pumped into the stratosphere. The newer the aircraft, the more efficient it is, but don't think you're doing the world a favour by flying to Los Angeles on a hi-tech MD11 - it's cleaner to go on a cycling-and- camping holiday at home.

H is for hand baggage. To avoid the risk of your socks ending up in Stockholm while your feet are in Phoenix, don't check-in luggage. Also, the chances of a quick getaway or a tight connection are higher if you don't have to wait for your luggage. The maximum weight for a piece of hand luggage is 5kg, the maximum dimension must not exceed 55cm, and the maximum single irritation for check-in clerks is passengers who try to exceed the limits. Australian airports have frames at check-in to reduce arguments - if your bag fits in, and is light enough, you can take it on board. Aeroflot even lets you wheel on a bicycle.

I is for Iata, the International Air Transport Association. The airlines' cartel, its main purpose is to fix fares; but fortunately for the passenger it fails miserably. While paying subscriptions and signing agreements with one hand, its member airlines are busy selling cheap tickets with the other. 'Iata is the reason we exist,' says a bucket-shop manager. The cartel used to busy itself with in-flight details as well, such as dictating the fee charged for renting the horrid plastic stethoscopes that passed as in-flight entertainment. Now, apart from fruitless fare-fixing, Iata does little more than designate airport codes.

J is for JFK. New York's Kennedy airport is the world's most oppressive, but has the best-known Iata acronym. Most Iata city codes bear a reasonably close relation to the place name, from ATL (Atlanta) to ZRH (Zurich). But if you're flying down to Rio, don't worry when your bag is tagged GIG. Some North American codes are bizarre: YYZ - Toronto, MSY - New Orleans, ORD - Chicago.

The last one earned its initials in the days when nothing bigger than a DC-3 landed there and it was called Orchard Field. Now it is the busiest airport in the world. Irritate a travel agent by asking for an excursion to Excursion (EXI, Alaska) or two Batman returns (BAL, Turkey).

K is for kids. Up to the age of two, children can travel for 10 per cent of the adult fare. Under-12s enjoy a 50 per cent discount on first, business and full economy fares, but pay proportionately more for lower fares. Perhaps the system used on Thai Railways could be introduced, where the division is not by age but height: if you're under 4 ft 6 in you pay half-fare.

L is for luggage, which is a pain: carrying it, paying excess for it, losing it. The most generous allowances are on transatlantic flights, the meanest on charter services. Each kilo of excess baggage is charged at 1 per cent of the first-class fare, except on transatlantic flights, when you pay pounds 50 a case. Probably the most irritating air travellers' habit is to check in bags, then disappear to the dark recesses of the duty-free shops, causing the aircraft to miss its take-off slot; aircraft cannot depart carrying bags whose owner is not on board.

This rule does not seem to trouble Continental Airlines: I travelled from LA via Denver and New York to London completely independently of my luggage, which turned up four days later.

M is for meals. To get a special one - vegetarian, kosher, salt-free - call at least 24 hours in advance. Air travel makes me hungry, so I routinely ask for second helpings. The most generous carrier was British Island Airways, which once gave me four meals between London and Nice but went out of business soon after. The meanest was Pan Am, which made a huge fuss about an extra biscuit between Frankfurt and Berlin; despite its frugality, it also went bankrupt.

N is for nerves. Two BA pilots take the fear out of flying on one-day courses at Heathrow and Manchester that have so far converted 10,000 to the joys of aviation, without resorting to gin and Valium. Captain Peter Hughes spends the first part of the day confronting the manifold fears of petrified punters. 'It's a very stressful morning,' he says, 'in which we deal with the mechanics of flying and our psychologist helps ease the anxieties that people feel.' Then the participants are put on an aircraft to test their nerves in flight.

Hardly anyone on the course balks at getting on board, but sometimes the flight itself is a little too realistic in displaying the full panoply of the stresses of air travel. The last 40-minute flight was held over Manchester for a further 40 minutes by air traffic control, while a previous trip was struck by lightning in mid-air. The price at Manchester is pounds 95, at Heathrow pounds 129. Contact Aviatours on 061-832 7972.

The statistic that keeps me flying is that cycling three miles across London is more dangerous than flying to LA.

O is for overbooking. Because full-fare passengers can fail to turn up for a flight without a penalty, they often book themselves on several flights and take whichever turns out to be the most convenient. Clearly, airlines would lose revenue unless they accepted more reservations than there are seats. Sometimes they guess wrong and everyone turns up. This is not only embarrassing, but also expensive - the European Commission lays down rules for denied-boarding compensation. So the airlines ask for volunteers: on a busy Friday afternoon at Heathrow, BA has been known to offer pounds 125 in cash for Frankfurt-bound passengers prepared to take a later flight.

The other weapon at the airlines' disposal is to enforce the latest check-in time strictly; if you arrive late, you forfeit all your rights.

P is for pregnancy. Most airlines carry women up to 27 weeks pregnant without formality. From 28 to 35 weeks inclusive, a doctor's letter is required. Beyond 36 weeks, permission is usually refused. Before flying to places such as La Paz (13,000 feet), be warned that the risk of miscarriage increases with altitude. Aircraft are pressurised to the equivalent of 8,000 feet.

Q is for Qantas, the safest airline in the world; no passenger has ever been killed on a Qantas flight. The Australian carrier also has commendably honest reservations staff. 'We're not allowed to offer fares as low as you can get from discount travel agents,' said one, 'so I'd advise you to contact an agent rather than booking direct.' One suitable source of cut-price Qantas tickets is Worldwide Travel (0493 440239).

R is for refunds. Full-fare ticket holders can get all their money back, but the cheaper the fare the more heavily restricted are refunds and changes. Be sure to find out the penalties before you buy.

What if the airline goes bust? Charter-flight passengers are heavily protected through the Air Transport Operators Licence scheme and the Air Travel Trust. Passengers on scheduled airlines are more exposed. To minimise the potential catastrophe, buy your ticket with a credit card.

S is for smoking. The airborne smoker is in danger of being stubbed out. Smoking is banned on domestic flights in Australia, Canada and the continental US. All Air UK scheduled services, and BA domestic flights are non-smoking. Nicotine chewing-gum is a poor substitute.

T is for taxes. Many governments see air travellers as easy pickings. If you can afford an air ticket, they theorise, you're obviously rich enough to be taxed. Passengers have to stump up cash to leave many Third World countries. Elsewhere, taxes are collected by the airline and added to the price of a ticket. Taxes to and from the US are the most mysterious. There the government imposes fees to pay for Customs, immigration and security, but the amount passed on to the customer varies from pounds 9.30 (BA) to pounds 20 (Virgin Atlantic).

U is for upgrades. Airport staff have considerable discretion to put passengers in a higher class of travel. The best candidates are full-fare economy travellers. Next in the queue for the front of the plane are executive club members. You will be upgraded only if you look reasonably smart. Some frequent-but- impecunious flyers carry a jacket and tie for precisely this purpose.

V is for visas. Most countries do not require British citizens to have visas, but a surprising number of Commonwealth countries do. India has strict and expensive visa rules (mirroring the impediments Britain puts before Indians). Kenya has a policy whereby British visitors of Asian origin need visas. The toughest visa to get is for Guyana: for the only Commonwealth country in South America you must undergo a thorough grilling and show you have savings of more than pounds 1,000. Like most visas, the Guyana stamp merely entitles you to apply for admission. If you don't impress the guy at Georgetown, you could be on the next flight out.

W is for world traveller. This is BA doublespeak for economy class on long-haul flights.

X is for X/O. Airline tickets seem deliberately mysterious. The column on the extreme left marked 'X/O' indicates whether a stopover is possible at each intermediate point. A bucket-shop ticket on Northwest to New York via Boston will be marked 'X' next to Boston, to prevent you stopping off there. A round-the-world itinerary should have 'O' beside each city.

Y is for youth and student fares. These are still the biggest bargains. If you are under 26 or a student, contact specialists such as Campus Travel, Council Travel, STA Travel or Travel CUTS for fares on scheduled airlines that undercut other bucket-shop prices. Some travellers can't see why, if an airline is prepared to sell tickets at these prices, fares should suddenly go up on their 26th birthday. Some agents sympathise, and supply illicit (and illegal) student cards.

Z is for zonal fares. Large chunks of the world are divided into zones for the purposes of fare calculation. Continental Airlines, for example, regards the whole of Central America as a single zone and doesn't care if you fly into one country and back from another. So for just pounds 488 (through South American Experience 071-976 5511), you could fly into Belize and back from the Costa Rican capital - if you know the way to San Jose.

(Photograph omitted)