If you have ever booked a flight on one airline and earned miles in a completely different airline's program, you have used an alliance. The three global airline alliances, Star Alliance, Oneworld and SkyTeam, link dozens of airlines into networks that share routes, lounges and loyalty benefits. For points travellers, alliances are the key to flying almost anywhere on earth with miles from a single program.

Star Alliance is the largest, with members including United, Lufthansa, Air Canada, ANA, Singapore Airlines and Turkish Airlines. Oneworld includes American Airlines, British Airways, Cathay Pacific, Qantas and Japan Airlines. SkyTeam includes Delta, Air France-KLM, Korean Air and Aeromexico. Between them, the three alliances cover nearly every major route on the planet.

The practical magic is that miles earned in one member's program can usually be redeemed for flights on any other member. That means United miles can book a Lufthansa flight, British Airways Avios can book a Japan Airlines flight, and Delta miles can book an Air France flight. You are not limited to the airline whose program you joined, which multiplies your options enormously.

Alliances save points because different programs charge different prices for the same flight. A flight operated by one airline might cost 80,000 miles in its own program but only 50,000 miles when booked through a partner program in the same alliance. Smart travellers compare prices across alliance partners before booking, and the savings can be dramatic.

Partner availability is the main limitation: airlines release only a certain number of award seats to partners, and popular routes can be hard to book. But with flexibility on dates and a willingness to check several programs, alliance bookings regularly beat booking directly. Tools that search multiple programs at once make this comparison much easier than it used to be.

Alliances also share elite status benefits. Gold status in one Star Alliance program generally gets you lounge access and priority boarding on any Star Alliance flight, whatever airline operates it. That means your loyalty to one airline pays off across dozens of others, which is a strong reason to concentrate your flying and earning in a single alliance.

For most travellers, the best strategy is to pick the alliance that serves your home airport and your favourite destinations best, then focus your earning there. Learn which partner programs offer the cheapest awards on your most common routes, and keep a note of those sweet spots. Alliances turn a single loyalty program into a global network, and understanding them is what separates casual points collectors from serious travel hackers.

One practical tip is to learn the quirks of the partner programs in your chosen alliance, because they are often very different from the airline you fly most. For example, some partner programs charge far fewer miles for the same flight, some allow free stopovers while others do not, and some have much lower taxes and fees on award tickets. A flight booked through one partner might cost 20,000 fewer miles and 200 dollars less in fees than the same flight booked through another partner, simply because of how each program prices awards. Keeping a short note of these differences for your most common routes turns alliance knowledge into real savings, and it is the kind of detail that separates a good redemption from a great one.

Airline alliances let you earn with one airline and fly with dozens. Learn the three alliances, compare partner award prices before booking, and concentrate your earning in the alliance that serves your routes best. That is how a single points balance opens up the world.