Brass instruments such as the trumpet and trombone usually play tones from the second and third octaves of the harmonic series, that is, the second through eighth harmonics (french horns play way up in the third and fourth octaves because they're just weird). But on the trombone, baritone, or tuba it is possible to play the fundamental, or first harmonic, which will be (if played correctly) a deep, resonant tone a full octave below all the other pitches that can be produced with that fingering or slide position. This is called a pedal tone. Pedal tones are difficult to impossible to play on the trumpet because the shape and size of the mouthpiece is intended to facilitate the playing of higher harmonics.