Sentence:  Each valve or valve combination forms its own <WSD>natural harmonic</WSD> series; however, every valve instrument is by nature out of tune with the equal tempered scale. That is, certain pitches within the <WSD>natural harmonic</WSD> series are out of tune relative to the pitches used in equal temperament. 
Meaning of the word marked with <WSD> tags:  The series of pitches produced by a wind instrument without modulation by a mute, valves, or keys.