You can also just use normal headphones, but not push them in all the way into the socket. You'll only get mono (through both speakers, but only a single channe

You can also just use normal headphones, but not push them in all the way into the socket. You'll only get mono (through both speakers, but only a single channel), but if you don't have an adapter it works and your headphones are probably a lot better than what's on the aircraft. The third pin is to provide power for active-noise cancellation headphones. Most airlines usually only give them out in business and above now.