A transposing instrument is an instrument that is not constructed in the key of C. Therefore, when a concert (middle) C is played on these instruments, it is actually another note.

Some instruments transpose by octave; some by smaller, diatonic intervals; and some instruments don't transpose at all. Instruments that transpose by less than an octave are considered to be in a different key. As a composer or arranger, a good way to remember common transposition method is "to hear its key you must write a C."

Common transposing instruments and their keys are:

• Clarinet - Bb
• Alto Saxophone - Eb
• Tenor Saxophone - Bb (8ve)
• Horn - F
• Trumpet - Bb
• Double bass - Sounds one octave lower
• Guitar - sounds one octave lower

This is by no means a comprehensive list. The transposition of an individual instrument either should be or will eventually be incorporated into that instrument's individual node.