The Volbox is an in-line audio volume adjuster that makes volume control quick and easy. It has a serrated knob that is nice to grip and smooth when turned. You can keep it in a spot that's easy to reach, or mount it somewhere out of sight. The stereo input and output jacks are the common 3.5mm (1/8") size. It works great as an inline headphone adjuster. It can be used with aux jacks or as an attenuator too. No batteries are required. The Volbox is hand built with good quality parts to provide many years of service. It is built inside a tough ABS case that's held together with screws. It measures about 2" x 1 3/8" x 1/2" tall (50mm x 35mm x 11mm). The knob is 3/4" x 1/2" tall (20mm x 12mm). Note: This will NOT make the volume louder than the music player's maximum level. For that you would need an amplifier. No cables are included. The Volbox includes a brief instruction card and uses minimal packaging to reduce the impact on our planet. Please contact me through Amazon at any time if you have questions about using the Volbox in your system and I will be happy to help you. Please remember, audio cables are not included. -Clark