One of the elements in the set of notes which constitutes a chord.
In familiar tertian harmonic theory, the generic names of the chord-members correspond to interval-names as follows (listed analagously to their pitch):
(15th = double-8ve) 13th 11th 9th 7th 5th 3rd root
Since traditional music-theory assumes octave-equivalence, and the '15th' is a double-octave duplication of the root, there are only 7 different chord-members possible in tertian harmony, excluding chromatic alteration of any of these 7; it is thus a closed system in this sense.
Just-intonation harmonic construction, on the other hand, is not limited in this way, since no cycle of any just ratio ever returns exactly to the pitch of the origin or to an 'octave'-duplication of it; just intonation is thus an open system in this sense.