What does it mean to transpose a Bb instrument?

Transposing a Bb instrument involves understanding its relationship to the concert pitch. When a Bb instrument, such as a Bb trumpet or Bb clarinet, plays a written C, it actually sounds a Bb in concert pitch. This means that the instrument is tuned such that when a musician plays a note that is notated as C, the pitch produced is a whole step lower, resulting in a sounding Bb.

To clarify, this is distinct from the sounding pitch of other instruments, such as C instruments, which play and sound the same note. The Bb instrument's transposition creates a necessary shift in notation to align with the desired concert pitch.