In what key should trumpet play and what key should saxophone play in order for a harnomy to happen?

To create harmony between a trumpet and a Saxophone, the trumpet typically plays in B♭ major, while the Alto Saxophone, which is an E♭ instrument, plays in the same concert key of B♭ major. This means that when the trumpet plays a written C, the alto saxophone would need to play an A to match the pitch. For a Tenor Saxophone, which is also in B♭, it can play in the same key as the trumpet without any transposition, allowing for easy harmonization.