There are two common instruments that are in the key of Eb. The first is the Eb alto saxophone, and the other is the Eb baritone saxophone. Being in the key of Eb means that when these instruments read a written C, an Eb concert pitch is heard. The Eb alto saxophone transposes a major sixth away from where it's written. This means that a melody written in concert pitch would have to be transposed up a major 6th in order to sound correct on the alto saxophone.

The baritone saxophone is also in the key of Eb; the only difference is that the baritone is a full octave below the alto sax, so it transposes at the intervals of a major sixth and an octave (or a major 13th). In order for a melody written in concert key to sound correctly on a baritone sax, it must be written a major 13th up! Remember, it sounds down, but it must be written up.