Controller keyboards, often used interchangeably with MIDI keyboards, are electronic musical keyboards designed primarily to send MIDI (Musical Instrument Digital Interface) data to other devices. Unlike traditional keyboards or digital pianos, controller keyboards typically don't produce sound on their own. Instead, they act as an interface, allowing musicians to control virtual instruments in software on a computer, hardware synthesizers, and other MIDI-compatible devices. They come in various sizes and often include additional controls like knobs, faders, and pads to provide greater flexibility and expression in music production and performance.