These 2.5mm male to 3.5mm female audio patch cables can be used to convert any speaker, headset or other device from a 2.5mm to a 3.5mm connection and vice versa. Since they are also male to female, they can be used as an extension and converter cable at the same time.

The 2.5mm connector is the smaller cousin to the  3.5mm, which is just a little larger in diameter.