Certain musical instruments play in a certain key, or have their music written in a certain key. Instruments that do not play in the key of C are known as transposing instruments. The most common kind of clarinet, for example, is said to play in the key of B. This means that a scale written in C major in sheet music actually sounds as a B major scale when played on the B-flat clarinet—that is, notes sound a whole tone lower than written. Likewise, the horn, normally in the key of F, sounds notes a perfect fifth lower than written.