Before we think deep thoughts about the practical and existential implications of airlines "unbundling" their fares and selling all manner of products and services on an a la carte basis, consider this hardheaded assessment from a plainspoken airline executive I know:

"Airlines wouldn't be unbundling fares if they didn't think they they'd generate additional revenue," he told me last week. "This is about money."

Now that you understand the chronically unprofitable airlines' motivation for suddenly charging you for everything from checking a bag to choosing a less-uncomfortable seat in coach, we can get to those deep thoughts:

• Does a la carte pricing actually raise more revenue for airlines than traditionally bundled fares?
• Can airlines track these new revenue streams on a line-item basis so they know which fees are working and which aren't?
• Do flyers have a fair basis for comparing the total cost of flying from one airline to the next?
• Does the federal government have a role in requiring carriers to display prices in a way that allows passengers to understand the total cost of flying?
• Are passengers defecting to carriers like Southwest and JetBlue, two airlines that minimize their reliance on ancillary revenue from items that were historically part of the airfare?
• Are airlines using unbundled pricing as a dodge to raise the price of travel for companies that have negotiated fares and contract discounts?
• Are airlines using a la carte pricing to duck taxes?

As I say, deep stuff for the usually languorous summertime, when the living is supposed to be easy and business travelers prefer not to think about business travel. It's all so convoluted and internecine that we have to start at the literal beginning: the definition of a "fare."

A generation ago, an airfare was an all-inclusive purchase. Except for a very few and very specific exceptions, when you bought your ticket, the airfare covered everything: your passage; an assigned seat; in-flight beverages, snacks and/or meals; carry-on and checked baggage; the right to change or cancel your flights without penalty; the right to use the payment method and the ticket-purchase venue of your choice; and all government-imposed taxes and fees.

But as the deregulated airlines increasingly competed on price instead of service, the idea of all-in prices began to erode. First, airlines invented fees for changing your reservations or canceling your tickets. Then they separated some government-mandated charges from the ticket price. As the price of oil began to spiral, airlines invented the fuel surcharge, an add-on levy atop the quoted price. Airlines began reserving the "best" seats in coach and "early boarding" privileges for its most frequent flyers. Inevitably, however, airlines realized they could charge the hoi polloi for those perks. Then came charges for purchasing a ticket at the airport or via telephone. Some airlines even charge you for using a credit card when you purchase tickets at the carrier's proprietary website. Free meals disappeared, and airlines began peddling sandwiches and munchies.

The trend to a la carte pricing really took off when most airlines decided your "fare" did not include your luggage. So most charge for checked bags now, and, starting next month, Spirit Airlines will impose a fee for some carry-ons too. And there's an insidious new fee you don't even know about: the day-of-travel surcharge. What's that? It's an additional post-fare mandatory extra charge for flying on certain "peak volume" days.

Long story short: As far as anyone can tell, your "airfare" on most carriers now includes only the actual passage. And therein lies the first a la carte trip point: The word fare has both contractual and tax-code implications.

Companies small and large have negotiated volume-discount contracts with airlines. Virtually all of those deals are based on either an unpublished, set-price fare or a percentage-off discount against the prevailing published fare. But almost no contracts give companies discounts on any fees or surcharges that the airlines impose. That means fuel levies, baggage charges, day-of-travel surcharges, and anything else an airline creates is exempt from corporate discounts. That has naturally infuriated corporate buyers, who feel airlines are engaged in backdoor price increases.

The unbundled pricing regimen has also played havoc for the tax structure on airline tickets. All fares include a 7.5 percent federal excise tax that is diverted to the Airport and Airways Trust Fund, which is used to pay for improvements to the national aviation infrastructure. But guess what? Surcharges and fees are not taxed. A Government Accountability Office study released this month claims that airlines collected $2.5 billion from checked-bag fees in fiscal 2009. Had that revenue been part of the fare subject to the federal excise tax, the GAO concluded, it would have meant an extra $186 million for the aviation trust fund.