Recently, we got an email from Ken, who wrote that he had bought a round-trip ticket from Atlanta to Kuwait from Northwest Airlines, operated by KLM, for the bargain basement price of $1175 round trip (this fare usually starts at $1500). Knowing that a low fare like this would be heavily restricted, he assumed he'd pay a penalty plus the difference in the fare if he needed to change his flight dates. But he didn't expect to be told, "Use it or lose it" by the airline when he tried to make a change, over a week in advance, to fly a day later than his ticket was scheduled. As in: No changes, not even for a penalty, and if you purposely "miss" your flight? No stored value for you. (Incidentally, there were seats available at the same price on the later flight.)

We contacted NWA's call center to ask about the flight and were told, "Well, it was a 'T' fare, so of course he couldn't store the value." Actually, this isn't quite accurate. We checked with NWA spokesperson Michelle Aguayo-Shannon, who confirmed that, while T class fares are usually quite restricted, they're not always "use it or lose it." The lesson here: While it would have been smart to thoroughly check the restrictions first, one class of airfare doesn't always hold the same restrictions within even a single airline. And one airline's "T" fare class could be another airline's "K."

Sound confusing? It can be. And it's certainly possible to simply cruise through airfare booking, never learning a thing about fare codes. But there are a couple of reasons to familiarize yourself with the basics: Anecdotal evidence is that, with airlines cutting capacity and in a constant state of financial flux, you'll be seeing more fares, like Ken's Northwest Airlines fare, that come with far heavier restrictions. Naturally, the most restricted airfares almost always come with the most attractive prices. But if you're always succumbing to price alone, you can miss out on some benefits that come with certain fare classes, such as bonus mile offers, elite status miles, special promotions, or quirky upgrades (which we'll get to later).

Why not just sell first and coach class tickets, and call it a day?

Before we get into the intricacies of class of service codes, here's a little background. Of course you know of the major classes of airline service: first, business, and economy. Those classes are subdivided into a variety of sub-classes: restricted business, full-fare economy, discounted economy, deeply discounted economy, etc., based on restrictions. A full-fare economy ticket will have fewer restrictions, such as advance purchase, minimum stay, or penalty-free refund than a discount economy ticket, but you'll pay for the privilege.

That's the reason you could find yourself sitting next to someone in coach who paid $200 less for his seat than you did, while waiting the same number of infuriating hours on the tarmac and buying the same overpriced snack boxes as you. But we digress. It's all part of inventory control. In order to stay profitable, years ago airlines began subdividing their seats, allocating a certain number of seats (or a "bucket") at each fare level per flight. The number of these seats depends on complicated formulas that factor in the route, the time of year, the expected breakdown of leisure vs. business passengers, and the time of day, among other things. The inventory control department will release certain "buckets" at different times, tightening or loosening the spigot as needed to capture as many potential passengers paying as much as possible. And no, the airlines don't make public how many buckets they've created in any subcategory.

What fare codes look like

The letter that denotes class of service is only one in a string of letters and numbers the airline puts together to describe the fare you've bought. You'll find the fare basis code in the fare basis box on a physical ticket, or on most e-ticket confirmations. Here's an example: Say you bought a ticket with the fare basis code KL14LNR. The letter K refers to the class of service for booking; the L refers to low season; the 14 refers to a 14-day advance booking; and the NR means non-refundable. More than one fare may exist for each class of service. For example, there might be two "K" fares - one for midweek travel and one for weekend travel.

Searching by class

The first letter of the string is the one you'll want to search for. Generally, first class fares are coded as F or P, business class is C or J, and full-fare coach is Y. After that, economy class fares run the gamut of alphabet letters, with the hierarchy varying from one airline to another. Here's a chart of major airlines' codes

Next, we'll tell you all about how to search for airfares by fare class, why and how to buy Q-UP, Y-UP, and Z-UP upgradeable airfares.