A transposing instrument is a musical instrument that does not play the notes you might think it will play. But all the notes differ from the real notes by the same musical interval. So a song played on the transposing instrument will sound familiar, but played in a different key. That is because the transposing instrument is tuned above or below what the usual notes would be, and always above or below by the same number of notes on a scale. The usual notes are called "concert pitch". Most non-transposing instruments, like pianos, are tuned to play in the key of C.