A twin-stick shooter is a subgenre of shoot 'em up video games. It is a multidirectional shooter in which the player character is controlled using two joysticks: one for movement on a flat plane, and one to aim and fire shots at enemies, Usually shots are fired as soon as the second joystick is moved, but in some games there is an additional button which must be held. Keyboard and mouse or touch input can supplant one or both of the joysticks when an arcade machine or controller is not used. Virtual joysticks can be used when a game is played on a touch screen.