Dear Cashing In,

I need a basic primer to understand how different air rewards cards compare to each other. As I understand it, with a general rewards card, I can use the miles to book a flight with any airline. True? If I have 75,000 miles, will they spend the same on any airline? Or does one airline require 35,000 miles and another require 50,000 miles? And, more importantly, if I accumulate 75,000 miles on a non-airline-specific card, would it apply to a flight to Alaska in the same way as using miles accumulated on an Alaska rewards card? Thank you for your help in understanding how these work.

- Nancy

Dear Nancy,

To answer your first question first: Yes, your miles on a generic rewards card, such as the Capital One Venture card, can be used on any airline and with no blackout dates. But their value is calculated differently than standard frequent flier miles.

With frequent flier miles, the kind accrued via an airline credit card, you purchase seats directly with your miles. For example, 25,000 frequent flier miles may be enough to get you one domestic round-trip economy airfare. Prices increase as the most desirable flights, based on times and dates, begin to fill up -- this bumps their value to the next “tier.”

Major airlines all share the same tier system. So, unless limited seat availability pushes up the value of your seat to the next tier (from the 25,000-mile, low-tier to the 40,000-mile, mid-tier, for example), you’ll find a pretty consistent mile value across airline loyalty programs.

It is, however, getting harder to find a seat on any of those airlines at that first tier. I’m told by the folks who take miles donations, such as the Make-A-Wish Foundation, that booking a domestic flight actually takes an average of 50,000 miles -- and they book thousands every year. So, while a flight can cost as little as 25,000 miles, it’s worth keeping in mind that the real price for one with reasonable restrictions may come in at double that or more.

Now, generic miles can be used on flights for any airline -- including smaller regional ones like Alaska Airlines -- but they have a direct dollar-to-mile value. The “miles” a Capital One Venture card accrues are really more like reward points that you happen to be able to spend on airfare; a mile in this case equals a penny. That means the 75,000 miles you accrue on your Venture card will equal $750 towards airfare. Those miles appear in your account as a credit you can spend, and the airlines you spend them on treat them like cash.

You earn two miles per dollar spent on the Venture card. This is equal to the amount earned on a typical airline credit card -- as long as you’re using the card to book flights on that airline. For other purchases, your airline card usually only gets you one mile per dollar. Thus, miles tend to pile up faster on generic rewards cards. On first glance, then, it doesn’t appear that the generic miles buy as much air fare; but if you’ve done the tier climb a few times, unable to redeem your miles and having had to shell out cash for a ticket anyway, that flat penny-per-point formula may start to look more attractive.

You can also spend generic miles on other travel expenses, not just airfare. That’s something to consider if you’re not a serious frequent flier. People who benefit most from conventional airline loyalty programs -- and the credit cards attached to them -- are those who spend serious time in the air. They can really pile up those miles and then save big bucks when using them to purchase, say, international airfare. If you don’t fit in that category, a generic miles card may work better in the long run.