Whether to buy a B fare is the question.

There are two “full economy” fares in the airline alphabet, B and Y. B is the lower of the two, Y is the higher—and you’re right, that sounds confusing. But keep the distinction in mind because full fares have the following advantages:

Here’s a rundown of how these fares play out on each of the Big Six US carriers:

‍American American offers low B class fares to several cities in Europe. Example: Say you’re traveling off season from Dallas to Dublin. It takes 50,000 miles to upgrade to Business Class from the lowest coach fare, $588, plus there is the $500 upgrade “surcharge” that’s imposed when upgrading from the lowest fares. But consider Plan B: The B class fares on this route can be as low as $917, so you pay $329 more but you’ll save$500 when you upgrade to Business Class by avoiding the upgrade surcharge and you save 30,000 miles (only 20K are required with B fares). So you’re $171 ahead; you’ve saved enough miles for your next domestic upgrade and the B fare will net you a 150% mileage bonus (13,392 vs. 8,928) toward elite status. Two caveats: American’s reservationists are not always hip to offering 20,000-mile upgrades with deeply discounted B fares, so you may have to be persistent or ask for a supervisor. The lowest fares are not available in all markets, meaning you’ll need a good travel agent or reservationist who can help you find them. (See page 3 for more fares and routes.)

Continental This carrier designates its full fares in H class, not B. It earns 150% bonus elite mile sand starts at $1,350—often less than the cost of the lowest fares once you add in the whopping $900 round trip upgrade surcharge. So make sure to ask your reservationist to compare the difference between the various levels of upgrades.

Delta You can often find reasonable B fares (M, too) to Europe that will net you 150% elite credit, something to consider at year-end if you must take a flight to maintain your status. However, Delta only reduces mileage upgrades (by 40%) for fares booked in Y class, usually not a great value unless you’re flying unrestricted anyway.

Northwest There’s a big advantage to booking B fares on this carrier: It allocates more upgrade seats with this fare bucket than with lower published fares usually the difference between getting a seat or not. Here it makes sense to use 30,000 miles (versus the 60,000 required with lower fares) to get around-trip upgrade. That’s especially true when you’re inside the 50-day window and faced with a $6,000+ fare. Business Travelers: Look here first when booking on short notice or without a Saturday night stay.

United This carrier has unbelievably low B fares to some European cities, sometimes only $200 to $400 more than the lowest upgradeable fare. One example: The lowest upgradeable Washington/Dulles-Dublin fare in July is $843. The B fare is $1,056. United charges only 30,000 miles for a round-trip upgrade off a B fare, whereas it charges 60,000 with the lower fares, and B fares earn 150% elite credit. Here the $213 higher fare will save you 30,000 miles—enough for another upgrade or around-trip domestic upgrade!

US Airways This airline’s B fares to Europe especially London—are absolutely the lowest you’ll find, sometimes only $150 more than the lowest fare. Example: The lowest upgradeable Philadelphia-London fare (on a non-stop no less) starts at $388, and requires 60,000 miles to upgrade. Book a B fare, however, starting at only $618 and you’ll only have to spend 20,000 miles to upgrade. In other words, the 40,000 miles saved cost you a mere $230! Here your miles can go three times farther.

More Sample Round-trip B Fares Fares vary significantly by origin and destination; many fares listed are for off-season travel.

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