Quarter-tone instruments are musical instruments designed to play notes that are a half-step apart, allowing for the use of quarter tones. These instruments expand the traditional Western scale, which divides an octave into 12 equal parts, by adding additional pitches. This enables musicians to explore a wider range of sounds and expressiveness, often found in various world music traditions. Common examples of quarter-tone instruments include the oud, a stringed instrument from the Middle East, and the piano with specially tuned keys. These instruments are often used in contemporary classical music and experimental genres, providing unique tonal possibilities for composers and performers.