This is a compatible headphone unit for the WonderSwan, WonderSwan Color, and WonderSwan Crystal. A headphone unit is required to play music in stereo. The case is a 3D-printed compatible part, and while complete compatibility with all software is not guaranteed, it has been confirmed to work without issue with major titles such as Final Fantasy and Digimon. (Operation confirmed) The connection terminals on the main unit are prone to poor contact due to aging, so cleaning them beforehand is recommended for better recognition. If you previously expressed interest in purchasing a power supply unit or case set, or if you have purchased one before, please contact me for a discount.