From Academic Kids
Like the horn, the mellophone has three valves. However, the valves are pressed with the right hand for mellophone, instead of the left for a concert horn. Also, although some of the mellophone fingerings are the same as those of the concert horn, they generally resemble trumpet fingerings more than horn fingerings. Mellophones are typically pitched in the key of F, like concert horns.
The main reason that the mellophone is used in place of the concert horn for marching is that the mellophone is a bell-front instrument, so that the sound goes in the direction that the player is facing. This is especially important in drum corps-style marching, in which the audience is typically standing or sitting on only one side of the band. The direction of the bell, as well as the much-reduced amount of tubing (as compared to a concert horn) makes the mellophone look like a large trumpet. In fact, most mellophones use trumpet-style parabolic ("cup") mouthpieces rather than the smaller, lighter, conical ("funnel") mouthpieces used on concert horns.
Although the terms mellophone and marching French horn are often used interchangeably, these are actually two different instruments. The marching French horn is typically larger than the mellophone, plays in the key of B flat or F, and uses B flat or F French horn fingerings, and French horn mouthpieces.