14. The _major triad_ is a concord containing three different notes
which (octaves being disregarded) are identical with the first, third
and fifth members of the harmonic series (the second and fourth
members being negligible as octaves).
15. The _minor triad_ is a concord containing the same intervals as
the major triad in a different order; in consequence it is artificial,
as one of its notes is not derivable from the harmonic series.
16. _Unessential discords_ are those that are treated purely as the
phenomena of transition, delay or ornament, in an otherwise concordant
harmony.
17. _Essential discords_ are those which are so treated that the mind
tends to regard them as definite chords possessing roots.
18. A _key_ is an harmonic system in which there is never any doubt as
to which note or triad shall be the final note of music in that
system, nor of the relations between that note or chord and the other
notes or chords. (In this sense the church modes are either not keys
or else they are subtle mixtures of keys.)