The TRI-STATE Joystick Interface allows most of the games available for the ZX Spectrum to be played with a joystick such as one of the Competition Pro series or any Atari/Commodore compatible type of joystick.
The left hand socket is the one to use for playing Kempston compatible games and cursor key controlled games. These two standards operate simultaneously and automatically so no setting up in required.
The right hand socket is for most of the Sinclair games and simulates the 6-0 keys.
WARNING Do not plug/unplug the interface with the power on.
(ZX Spectrum not included)