Now, it’s easy to see why this is an attractive proposition. While it’s hard to place an exact value on airline miles (since an award flight on United that cost 25,000 miles to book may cost $300 to book as a paid fare, or $500 to book as a paid fare), a conservative estimate is generally 1.5 cents per mile, effectively giving you 1.5% cash back (for frequent flyer programs like Southwest, JetBlue, and Virgin America, it’s a little easier to calculate the value of miles since they correspond directly with the cost of the flight, i.e. if a flight costs twice as much in dollars, it will cost twice as much in miles).

So what’s the problem? Well, your primary credit card (assuming you’re not trying to meet the minimum spending bonus) should be one that gives you lots of opportunities where you can earn bonus points or miles. But with a few exceptions, most airline credit cards will only give you bonus miles for purchases with the airline. Unless you’re constantly on the road for work, it’s unlikely that most of your spending will be with an airline (and even if it is, it’s probably on the corporate credit card).

So how can you earn miles while still taking advantage of bonus categories? Thankfully, several of the big credit card companies allow you to earn points directly with them in a variety of categories that will allow you to then transfer those points (often at a 1:1 ratio) to many leading frequent flyer programs.

A classic example is the United MileagePlus Explorer Card vs. the Chase Sapphire Preferred Card. The former gives you double miles on all United purchases and 1 mile for every dollar spent on other purchases. The latter gives you double points on all dining and travel purchases (including United purchases), and 1 point for everything else. These points can then be transferred 1:1 into your United account (or a number of other accounts). The annual fees ($95 after first year) and signup bonus (50,000) are identical.

To put this into perspective, if you spend $12,000 in a year, $3,000 of which is on dining or travel (with a $500 purchase on United), then you would earn 15,000 Ultimate Rewards points, which you could then transfer into 15,000 United miles if you wanted to, but you’d also have the flexibility to convert them into 15,000 Southwest miles (or British Airways miles, or a number of other programs). If you spent that same $12,000 in a year, then you’d earn 12,500 United miles. It isn’t hard to see why cards like the Chase Sapphire Preferred is a better bet.

Of course, that doesn’t mean airline credit card should be ignored. They’re great for boosting your mileage accounts with their often-generous signup bonuses (I’ve received over 500,000 miles in signup bonuses on airline credit cards), and if you regularly check bags, they tend to allow you to waive checked bag fees for you and a partner, which could save $100 or so on a round trip. Another common benefit is priority boarding. Furthermore, as long as you have one open, it means your miles will never expire (though there are other ways to extend the life of your miles. They also sometimes will come with companion passes. In other words, feel free utilize airline cards for their perks, just don’t put any more spending on them after you hit the bonus.

Below, I’ll run through some of the more popular airlines and their cards, explaining other cards which will get you more miles, but why the airline’s credit card still could be useful to have:

United: As I mentioned earlier, the Chase Sapphire Preferred will give you double points on travel and dining, which can then be converted to United miles. However, the United MileagePlus Explorer is probably one of the better airline cards out there, as in addition to the generous signup bonus (50,000) it can give you access to more award seats and two lounge passes a year.

(The card featured in the image is indeed a real card, which I put $2,000 on to meet the requirements to earn 50,000 miles plus a $50 statement credit, and plan on never using again).

American: American is a bit trickier, as none of the major credit card programs (Citi ThankYou, Chase Ultimate Rewards, AMEX Membership Rewards) allow you to transfer points to American. However, you can transfer from a Starwood Preferred Guest account, which is extremely valuable because of the wide range of partners you can transfer to from it, and you can get a 25,000-point signup bonus with their co-branded AMEX card (though the impacts of the upcoming merger with Marriott are unclear). Of course, American does have some good co-branded cards, and the Platinum World Elite, in addition to a 50,000-mile signup bonus, will also give you 10% off award flights (from time to time you might also see a 100,000-point bonus for the Executive version of the card, which has a $450 fee but gives you access to the Admiral lounges).

Delta: The AMEX Premier Rewards Gold card earns bonus points in number of different categories like travel and groceries, and allows you to transfer points at a 1:1 ratio into your Delta frequent flyer account. The standard Delta co-branded card (Gold Rewards) occasionally offers 50,000-point signup bonuses, but doesn’t really offer much else beyond the standard priority boarding and free checked bag.

Southwest: The Chase Sapphire Preferred mentioned above will also allow you to transfer your points at a 1:1 ratio to Southwest. That being said, both Southwest cards do give you an anniversary bonus after payment of the annual fee, and can be a good way to work toward the companion pass, not to mention it often offers signup bonuses as high as 50,000 points, about $625 in travel.

JetBlue: Again, the AMEX Premier Rewards Gold card will allow you to transfer JetBlue points though at a 5:4 ratio (e.g 50,000 Membership Rewards points equals 40,000 TrueBlue points). There currently is no JetBlue card to apply for, as it is being transitioned from American Express to Barclaycard.

Virgin America: And yet again, the AMEX Premier Rewards Gold card will allow you to transfer points, though at a 2:1 ratio (though if you transfer by March 10, it goes up to a 4:3 ratio). If you have the Citi Premier or Citi Prestige card, you can also transfer ThankYou points at a 2:1 ratio also, both of which have good bonus categories on travel and entertainment. While 2:1 may sound like a bad deal initially, given how valuable Virgin America points are (roughly worth a little over 2 cents each), it’s actually not that bad in reality. Of course, given how valuable they are, if your main goal is to accumulate Virgin America Elevate points, their co-branded VISA card with Comenity may not be a bad option, other than the fact that Comenity is by far the single-worst issuer I’ve ever dealt with.

Alaska: Similarly to American, the only way to transfer in miles to your Alaska account at a 1:1 ratio is through Starwood Preferred Guest, which I would again recommend over the Alaska Airlines co-branded card for the flexibility. That being said, the Bank of America Alaska card is a nice way to get a quick 25,000 miles, as there is no minimum spending bonus, though the annual fee of $89 is not waived the first year. There is also a good deal for a companion certificate with this card.

Hawaiian: I’ve never seen anyone besides myself with the Hawaiian Airlines co-branded card, but just in case you’re thinking about it, you’ll again do better transferring in points from the AMEX Premier Rewards Gold card. The 35,000-mile signup bonus on the co-branded card is still a nice way to get a free roundtrip flight to Hawaii from the West Coast, not to mention there are companion certificate benefits too, though the annual fee is not waived the first year.

Spirit: Other than flying Spirit, there’s no other practical way to earn additional Free Spirit miles besides the Spirit Airlines credit card. But if your primary goal is to accumulate miles to use on Spirit, I question your life choices.

Did I miss something? Have a question about something? Feel free to e-mail me or post in the comments below.