What is what key is mellophone in?

The mellophone is commonly pitched in F and is considered a transposing instrument, meaning that the music is written in a different key than what it sounds like. When a mellophone player reads a concert pitch C on sheet music, they will play an F on their instrument. This is because the mellophone is a member of the brass family and is similar in shape and size to a French horn, but its tubing is longer, providing a lower pitch in F.