For a project, I need media keys (play, volume, mute, …) and a way to lock a computer under Windows (Windows+L keys). There is a lot of HID keyboard descriptors on the Internet, some are for keyboards + media but all contain at least one unused byte… Not a good thing for embedded systems with constraint memory. Why? The descriptor can be found in USB-IF example (Appendix B) and describe a boot compatible keyboard. So unless you need your keyboard in boot menus, this byte can be dropped. Also, the example defines 6 simultaneous keys, I decreased it to three as I don’t need more.