The Harmonic Series are the set of notes that can be played without the aid of valves. Before valves, these were the only notes that could be produced. In the harmonic series, the notes grow closer to each other as they become higher. For this reason, instrument makers before the invention of valves made trumpets of a longer length so that more notes in the series could be used. The harmonic series of a natural trumpet (without valves) is as follows:

Today's shorter trumpets move this series an octave higher, whch alters the series to look like this:

This series is also present in the form of overtones. Whenever a certain note is played, it is not the only frequency that is produced, but it is the strongest. Many of the note's overtones, or related notes in its series, are present. This means that if a trumpeter plays a middle G, the trumpet is producing the sound for G, C, E, and so on. The overtone series is what gives the trumpet its characteristc color, or timbre. When played at a soft volume, less overtones are present, so the notes are harder to hit and hold steady. When played at higher dynamic levels, however, many more overtones are present, and the notes are easier to hit and sustain. Generally, the best sounding players are the ones who can get the most overtones in their sound.