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. While it's not guaranteed to work perfectly with all software, it has been confirmed to work without issue with major titles like 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 will likely improve recognition. If you previously expressed interest in purchasing a power supply unit or case set, or if you've purchased from me before, please contact me for a discount.