While a double horn has a trigger that allows you to switch between F and B♭ (a fifth higher), the mellophone is a single horn that has no trigger. The mellophone is in F alto, a full octave higher than the French horn in F.  This is the same pitch as the F alto in a double descant or triple French horn. Some vintage concert mellophones have an E♭ trigger that allows you to go a full tone lower and which can be helpful with intonation issues. And the mellophonium used an E♭ crook for the same purpose.