Lots of folks have the right answers for old planes and plenty of folks have noted that newer planes are mostly switching to standard 3.5mm plugs. One thing nobody has mentioned is that new planes will sometimes have 2 prong plugs in first and business class for long distance international flights. The reason for this is that in first and business class, the seats often come with noise cancelling headphones and the second prong is used to power te noise cancelling circuitry. Typically on these newer 2 prong plugs, the audio prong is still a standard 3.5mm so you can use any headphones if you choose not to use the provided ones.