If you like to travel on a particular airline frequently, signing up for their mileage program awards you with points every time you fly. You can then exchange those points for free travel, hotel stays and merchandise.
Frequent flyer programs exist to encourage you to always fly the same airline. The more flights you take, the more miles you accrue and the closer you come to travel rewards.
Because of this, in general, frequent flyer miles from one airline cannot be combined or exchanged with those from another airline.
However, points earned among partner airlines of one alliance may be deposited into the mileage programs of any other member. Once deposited, points may not be exchanged, but award travel can be taken on any partner.
For example, the Star Alliance includes Spanair, BMI and United. If you fly on Spanair, you can deposit your earned miles into the United frequent flyer program. Once you accrue enough for a free flight, you can travel on BMI.
Broker sites such as Points.com allow exchanges of miles between their airline partners, even if they don't belong to the same alliance. Check out their website for a list of participating airlines.
