The fingering schema arises from the length of each valve's tubing (a longer tube produces a lower pitch). Valve "1" increases the tubing length enough to lower the pitch by one whole step, valve "2" by one half step, and valve "3" by one and a half steps. This scheme and the nature of the overtone series create the possibility of alternate fingerings for certain notes. For example, third-space "C" can be produced with no valves engaged (standard fingering) or with valves 2–3. Also, any note produced with 1–2 as its standard fingering can also be produced with valve 3 – each drops the pitch by steps. Alternate fingerings may be used to improve facility in certain passages, or to aid in intonation. Extending the third valve slide when using the fingerings 1–3 or 1-2-3 further lowers the pitch slightly to improve intonation.